The salinity, temperature, and delta18O of the glacial deep ocean

Dissolved Inorganic Carbon Evolution of Sediment Porewater in the Huixian Wetland, Southwest China

Wetlands, as crucial terrestrial carbon reservoirs, have recently suffered severe degradation due to intense human activities. Lacustrine sediments serve as vital indicators for understanding wetland environmental changes. In the current paper, porewater samples were extracted from lacustrine sediment in three boreholes with a depth of ~75 cm in the Huixian karst wetland, southwest China, to study the chemical and dissolved inorganic carbon (DIC) evolution under anthropogenic influence. Two boreholes are situated beneath the Mudong Lake, while the other one is in the degraded wetland area. The results show that porewater in the central region of Mudong Lake is natural HCO3-Ca type water and recharged by karst groundwater as evidenced by depleted 2H -18O isotopes. Methanogenesis prevails in this area, suggested by positive δ13C values ranging from 4.29‰ to 7.05‰. However, shallow porewater at the western edge of Mudong Lake and porewater in the degraded wetland exhibit significantly higher concentrations of NO3 - and SO4 2-, resulting from the agricultural input and recharged groundwater influenced by oxidation of pyrite. These processes lead to a decrease in methane production and generate DIC through degradation of organic fertilizer and carbonate weathering by sulfuric acid, thereby significantly altering porewater δ13C values. Two types of DIC mixing processes were observed based on the increasing δ13C values with depth, which can be attributed to the unique karst groundwater subsystems. This work highlights the potential impact of human-induced porewater chemical variations on the fate of DIC, particularly in karst wetland environments.

Glacial Southern Ocean deep water Nd isotopic composition dominated by benthic modification

The deep Southern Ocean (SO) circulation plays a key role in the storage and release of CO2 in Earth's climate system. The uptake and release of CO2 strongly depend on the redistribution of well and poorly ventilated deep ocean water masses. Recently, evidence was found for possible stronger Pacific deep water overturning and subsequent intrusion into the SO during periods of reduced AMOC. Here, we present new authigenic neodymium isotope data (ɛNd) from two sites within the Atlantic sector of the SO to assess the distribution of water masses during the past 150 ka. PS 1768-8 (3299 m) and ODP 1093 (3624 m) feature unradiogenic interglacial ɛNd-signatures, which are typical for present-day Weddell Sea sourced Antarctic Bottom Water (AABW) (ɛNd ~  - 8.6). During peak glacial periods, radiogenic ɛNd-values ranging from ~  - 2.5 to - 3.5 are recorded. This may be the result of either a strong Pacific or benthic flux influence on the Nd budget in the Atlantic sector of the SO. However, an ocean circulation model indicates no stronger Pacific influence during glacials. Thus, we suggest that an increase in benthic flux influences the SO Nd budget, which is modulated by ACC strength. The more stratified and more sluggish deep water supports decreased vertical mixing and increased glacial carbon storage without the intrusion of poorly ventilated Pacific waters. The occurrence of highly radiogenic glacial bottom water or porewater signatures requires reassessment of the glacial Southern Hemisphere ɛNd-endmember for water mass sourcing reconstructions in the glacial Atlantic.

A solution for constraining past marine Polar Amplification

Most climate proxies of sea surface temperatures suffer from severe limitations when applied to cold temperatures that characterize Arctic environments. These limitations prevent us from constraining uncertainties for some of the most sensitive climate tipping points that can trigger rapid and dramatic global climate change such as Arctic/Polar Amplification, the disruption of the Atlantic Meridional Overturning Circulation, sea ice loss, and permafrost melting. Here, we present an approach to reconstructing sea surface temperatures globally using paired Mg/Ca - δ18Oc recorded in tests of the polar to subpolar planktonic foraminifera Neogloboquadrina pachyderma. We show that the fidelity of Mg/Ca-based paleoclimate reconstructions is compromised by variations in seawater carbonate chemistry which can be successfully quantified and isolated from paleotemperature reconstructions using a multiproxy approach. By applying the calibration to the last glacial maximum, we show that marine polar amplification has been underestimated by up to 3.0 ± 1.0 °C in model-based estimates.

Enhanced ocean heat storage efficiency during the last deglaciation

Proxy reconstructions suggest that increasing global mean sea surface temperature (GMSST) during the last deglaciation was accompanied by a comparable or greater increase in global mean ocean temperature (GMOT), corresponding to a large heat storage efficiency (HSE; ∆GMOT/∆GMSST). An increased GMOT is commonly attributed to surface warming at sites of deepwater formation, but winter sea ice covered much of these source areas during the last deglaciation, which would imply an HSE much less than 1. Here, we use climate model simulations and proxy-based reconstructions of ocean temperature changes to show that an increased deglacial HSE is achieved by warming of intermediate-depth waters forced by mid-latitude surface warming in response to greenhouse gas and ice sheet forcing as well as by reduced Atlantic meridional overturning circulation associated with meltwater forcing. These results, which highlight the role of surface warming and oceanic circulation changes, have implications for our understanding of long-term ocean heat storage change.

Last Glacial Maximum pattern effects reduce climate sensitivity estimates

Here, we show that the Last Glacial Maximum (LGM) provides a stronger constraint on equilibrium climate sensitivity (ECS), the global warming from increasing greenhouse gases, after accounting for temperature patterns. Feedbacks governing ECS depend on spatial patterns of surface temperature ("pattern effects"); hence, using the LGM to constrain future warming requires quantifying how temperature patterns produce different feedbacks during LGM cooling versus modern-day warming. Combining data assimilation reconstructions with atmospheric models, we show that the climate is more sensitive to LGM forcing because ice sheets amplify extratropical cooling where feedbacks are destabilizing. Accounting for LGM pattern effects yields a median modern-day ECS of 2.4°C, 66% range 1.7° to 3.5°C (1.4° to 5.0°C, 5 to 95%), from LGM evidence alone. Combining the LGM with other lines of evidence, the best estimate becomes 2.9°C, 66% range 2.4° to 3.5°C (2.1° to 4.1°C, 5 to 95%), substantially narrowing uncertainty compared to recent assessments.

Evolution of Atlantic Meridional Overturning Circulation since the last glaciation: model simulations and relevance to present and future

The Atlantic Meridional Overturning Circulation (AMOC) and the associated water masses have changed dramatically during the glacial-interglacial cycle. Here, I review some recent progress in the modelling of the AMOC and water masses since the Last Glacial Maximum and discuss the relevance of these past AMOC studies to the present and future AMOC study. Recent studies suggested that Atlantic water masses were constrained by carbon isotopes (δ13C) and neodymium isotopes (εNd), while the strength of the AMOC better was constrained by protactinium/thorium ratio (231Pa/230Th) and the spatial gradient of calcite oxygen isotopes (δ18Oc). In spite of the shallower AMOC at the glacial period, its intensity did not differ substantially from the present because of the cancellation of opposite responses to the rising CO2 and the retreating ice sheet. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

A multicentennial mode of North Atlantic climate variability throughout the Last Glacial Maximum

Paleoclimate proxy records from the North Atlantic region reveal substantially greater multicentennial temperature variability during the Last Glacial Maximum (LGM) compared to the current interglacial. As there was no obvious change in external forcing, causes for the increased variability remain unknown. Exploiting LGM simulations with a comprehensive coupled climate model along with high-resolution proxy records, we introduce an oscillatory mode of multicentennial variability, which is associated with moderate variations in the Atlantic meridional overturning circulation and depends on the large-scale salinity distribution. This self-sustained mode is amplified by sea-ice feedbacks and induces maximum surface temperature variability in the subpolar North Atlantic region. Characterized by a distinct climatic imprint and different dynamics, the multicentennial oscillation has to be distinguished from Dansgaard-Oeschger variability and emerges only under full LGM climate forcing. The potential of multicentennial modes of variability to emerge or disappear in response to changing climate forcing may have implications for future climate change.

Unique thermal expansion properties of water key to the formation of sea ice on Earth

The formation of sea ice in polar regions is possible because a salinity gradient or halocline keeps the water column stable despite intense cooling. Here, we demonstrate that a unique water property is central to the maintenance of the polar halocline, namely, that the thermal expansion coefficient (TEC) of seawater increases by one order of magnitude between polar and tropical regions. Using a fully coupled climate model, it is shown that, even with excess precipitations, sea ice would not form at all if the near-freezing temperature TEC was not well below its ocean average value. The leading order dependence of the TEC on temperature is essential to the coexistence of the mid/low-latitude thermally stratified and the high-latitude sea ice-covered oceans that characterize our planet. A key implication is that nonlinearities of water properties have a first-order impact on the global climate of Earth and possibly exoplanets.

Evidence for late-glacial oceanic carbon redistribution and discharge from the Pacific Southern Ocean

Southern Ocean deep-water circulation plays a vital role in the global carbon cycle. On geological time scales, upwelling along the Chilean margin likely contributed to the deglacial atmospheric carbon dioxide rise, but little quantitative evidence exists of carbon storage. Here, we develop an X-ray Micro-Computer-Tomography method to assess foraminiferal test dissolution as proxy for paleo-carbonate ion concentrations ([CO₃^2-]). Our subantarctic Southeast Pacific sediment core depth transect shows significant deep-water [CO₃^2-] variations during the Last Glacial Maximum and Deglaciation (10-22 ka BP). We provide evidence for an increase in [CO₃^2-] during the early-deglacial period (15-19 ka BP) in Lower Circumpolar Deepwater. The export of such low-carbon deep-water from the Pacific to the Atlantic contributed to significantly lowered carbon storage within the Southern Ocean, highlighting the importance of a dynamic Pacific-Southern Ocean deep-water reconfiguration for shaping late-glacial oceanic carbon storage, and subsequent deglacial oceanic-atmospheric CO₂ transfer.

Geophysical and Geological Views of Potential Water Resources in the North-Eastern Adriatic Sea

The increasing demand for freshwater requires the identification of additional and less-conventional water resources. Amongst these, offshore freshwater systems have been investigated in different parts of the world to provide new opportunities to face increasing water requests. Here we focus on the north-eastern Adriatic Sea, where offshore aquifers could be present as a continuation of onshore ones. Geophysical data, in particular offshore seismic data, and onshore and offshore well data, are interpreted and integrated to characterise the hydrogeological setting via the interpretation of seismo-stratigraphic sequences. We focus our attention on two areas located in the proximity of the Tagliamento and Isonzo deltas. Well and seismic data indicate that the Quaternary sediments, that extend from onshore to offshore areas, are the most promising from an offshore freshwater resources point of view, while the several kilometres thick pre-Quaternary carbonate and terrigenous sequences likely host mainly salty waters.

Glacial carbon cycle changes by Southern Ocean processes with sedimentary amplification

Recent paleo reconstructions suggest that increased carbon storage in the Southern Ocean during glacial periods contributed to low glacial atmospheric carbon dioxide concentration (pCO₂). However, quantifying its contribution in three-dimensional ocean general circulation models (OGCMs) has proven challenging. Here, we show that OGCM simulation with sedimentary process considering enhanced Southern Ocean salinity stratification and iron fertilization from glaciogenic dust during glacial periods improves model-data agreement of glacial deep water with isotopically light carbon, low oxygen, and old radiocarbon ages. The glacial simulation shows a 77-ppm reduction of atmospheric pCO₂, which closely matches the paleo record. The Southern Ocean salinity stratification and the iron fertilization from glaciogenic dust amplified the carbonate sedimentary feedback, which caused most of the increased carbon storage in the deep ocean and played an important role in pCO₂ reduction. The model-data agreement of Southern Ocean properties is crucial for simulating glacial changes in the ocean carbon cycle.

Sea level and deep-sea temperature reconstructions suggest quasi-stable states and critical transitions over the past 40 million years

Sea level and deep-sea temperature variations are key indicators of global climate changes. For continuous records over millions of years, deep-sea carbonate microfossil-based δ¹⁸O (δ_c) records are indispensable because they reflect changes in both deep-sea temperature and seawater δ¹⁸O (δ_w); the latter are related to ice volume and, thus, to sea level changes. Deep-sea temperature is usually resolved using elemental ratios in the same benthic microfossil shells used for δ_c, with linear scaling of residual δ_w to sea level changes. Uncertainties are large and the linear-scaling assumption remains untested. Here, we present a new process-based approach to assess relationships between changes in sea level, mean ice sheet δ¹⁸O, and both deep-sea δ_w and temperature and find distinct nonlinearity between sea level and δ_w changes. Application to δ_c records over the past 40 million years suggests that Earth's climate system has complex dynamical behavior, with threshold-like adjustments (critical transitions) that separate quasi-stable deep-sea temperature and ice-volume states.

Combining Modern and Paleoceanographic Perspectives on Ocean Heat Uptake

Monitoring Earth's energy imbalance requires monitoring changes in the heat content of the ocean. Recent observational estimates indicate that ocean heat uptake is accelerating in the twenty-first century. Examination of estimates of ocean heat uptake over the industrial era, the Common Era of the last 2,000 years, and the period since the Last Glacial Maximum, 20,000 years ago, permits a wide perspective on modern-day warming rates. In addition, this longer-term focus illustrates how the dynamics of the deep ocean and the cryosphere were active in the past and are still active today. The large climatic shifts that started with the melting of the great ice sheets have involved significant ocean heat uptake that was sustained over centuries and millennia, and modern-ocean heat content changes are small by comparison.

Acute and chronic toxicity of nickel on freshwater and marine tropical aquatic organisms

Water quality guidelines and ecological risk assessment of chemical substances like nickel (Ni) in tropical regions such as South East Asia and Melanesia are often based on temperate information as a result of fewer Ni ecotoxicity data available for tropical species. This leaves an unknown margin of uncertainty in the risk assessment in the tropics. In order to fill this data gap, this study was designed to conduct standard toxicity tests on Ni with two freshwater species (acute tests) and three marine species (acute and chronic tests) originated from tropical Hong Kong. All tests were carried out using measured concentrations of Ni with control mortality below 15%. The median lethal concentrations (LC₅₀s) were determined as 2520 (95% confidence interval: 2210, 2860) and 426 (351, 515) μg Ni L^-1 for the freshwater gastropods Pomacea lineata (48 h) and Sulcospira hainanensis (96 h), respectively, while 96 h LC₅₀s of 4300 (3610, 5090), 18,200 (6470, 51,200), 62,400 (56,800, 68,500), and 71,700 (68,200, 75,400) μg Ni L^-1 were derived for the marine copepod Tigriopus japonicus, the gastropod Monodonta labio, juvenile and adult of the marine fish Oryzias melastigma, respectively. The chronic effect concentration of 10% (EC₁₀) based on the intrinsic rate of increase of the population of T. japonicus was 29 (12, 69) μg Ni L^-1. In terms of growth inhibition, the chronic EC₁₀ for M. labio was 34 (17, 67) μg Ni L^-1. The results also indicated that T. japonicus in maturation stage (LC₁₀: 484 (349, 919) μg Ni L^-1) was less sensitive than its nauplii stage (LC₁₀: 44 (27, 72) μg Ni L^-1). This study represents an important addition of high-quality toxicity data to the tropical Ni toxicity database which can be used for future ecological risk assessment of Ni and derivation of its water quality guidelines in tropical regions.

Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific

Although the Pacific Ocean is a major reservoir of heat and CO₂, and thus an important component of the global climate system, its circulation under different climatic conditions is poorly understood. Here, we present evidence that during the Last Glacial Maximum (LGM), the North Pacific was better ventilated at intermediate depths and had surface waters with lower nutrients, higher salinity, and warmer temperatures compared to today. Modeling shows that this pattern is well explained by enhanced Pacific meridional overturning circulation (PMOC), which brings warm, salty, and nutrient-poor subtropical waters to high latitudes. Enhanced PMOC at the LGM would have lowered atmospheric CO₂-in part through synergy with the Southern Ocean-and supported an equable regional climate, which may have aided human habitability in Beringia, and migration from Asia to North America.

Timing and magnitude of Southern Ocean sea ice/carbon cycle feedbacks

The Southern Ocean (SO) played a prominent role in the exchange of carbon between ocean and atmosphere on glacial timescales through its regulation of deep ocean ventilation. Previous studies indicated that SO sea ice could dynamically link several processes of carbon sequestration, but these studies relied on models with simplified ocean and sea ice dynamics or snapshot simulations with general circulation models. Here, we use a transient run of an intermediate complexity climate model, covering the past eight glacial cycles, to investigate the orbital-scale dynamics of deep ocean ventilation changes due to SO sea ice. Cold climates increase sea ice cover, sea ice export, and Antarctic Bottom Water formation, which are accompanied by increased SO upwelling, stronger poleward export of Circumpolar Deep Water, and a reduction of the atmospheric exposure time of surface waters by a factor of 10. Moreover, increased brine formation around Antarctica enhances deep ocean stratification, which could act to decrease vertical mixing by a factor of four compared with the current climate. Sensitivity tests with a steady-state carbon cycle model indicate that the two mechanisms combined can reduce atmospheric carbon by 40 ppm, with ocean stratification acting early within a glacial cycle to amplify the carbon cycle response.

No detectable Weddell Sea Antarctic Bottom Water export during the Last and Penultimate Glacial Maximum

Weddell Sea-derived Antarctic Bottom Water (AABW) is one of the most important deep water masses in the Southern Hemisphere occupying large portions of the deep Southern Ocean (SO) today. While substantial changes in SO-overturning circulation were previously suggested, the state of Weddell Sea AABW export during glacial climates remains poorly understood. Here we report seawater-derived Nd and Pb isotope records that provide evidence for the absence of Weddell Sea-derived AABW in the Atlantic sector of the SO during the last two glacial maxima. Increasing delivery of Antarctic Pb to regions outside the Weddell Sea traced SO frontal displacements during both glacial terminations. The export of Weddell Sea-derived AABW resumed late during glacial terminations, coinciding with the last major atmospheric CO₂ rise in the transition to the Holocene and the Eemian. Our new records lend strong support for a previously inferred AABW overturning stagnation event during the peak Eemian interglacial.

The Southern Ocean plays a key role in glacial-interglacial transitions and today, Weddell Sea derived Antarctic Bottom Water is one of the most important deep water masses. New records show that in contrast to today, no Weddell Sea water was exported during the last two glacial maxima, providing new insights towards the condition of Antarctic Bottom Water formation in extreme climate states.

The Biological Pump During the Last Glacial Maximum

Much of the global cooling during ice ages arose from changes in ocean carbon storage that lowered atmospheric CO₂. A slew of mechanisms, both physical and biological, have been proposed as key drivers of these changes. Here we discuss the current understanding of these mechanisms with a focus on how they altered the theoretically defined soft-tissue and biological disequilibrium carbon storage at the peak of the last ice age. Observations and models indicate a role for Antarctic sea ice through its influence on ocean circulation patterns, but other mechanisms, including changes in biological processes, must have been important as well, and may have been coordinated through links with global air temperature. Further research is required to better quantify the contributions of the various mechanisms, and there remains great potential to use the Last Glacial Maximum and the ensuing global warming as natural experiments from which to learn about climate-driven changes in the marine ecosystem.

Modulation of Indian monsoon by water vapor and cloud feedback over the past 22,000 years

To predict how monsoons will evolve in the 21st century, we need to understand how they have changed in the past. In paleoclimate literature, the major focus has been on the role of solar forcing on monsoons but not on the amplification by feedbacks internal to the climate system. Here we have used the results from a transient climate simulation to show that feedbacks amplify the effect of change in insolation on the Indian summer monsoon. We show that during the deglacial (22 ka to 10 ka) monsoons were predominantly influenced by rising water vapor due to increasing sea surface temperature, whereas in the Holocene (10 ka to 0 ka) cloud feedback was more important. These results are consistent with another transient simulation, thus increasing confidence despite potential model biases. We have demonstrated that insolation drives monsoon through different pathways during cold and warm periods, thereby highlighting the changing role of internal factors.

Equatorial Pacific seawater pCO2 variability since the last glacial period

The ocean may have played a central role in the atmospheric pCO2 rise during the last deglaciation. However, evidence on where carbon was exchanged between the ocean and the atmosphere in this period is still lacking, hampering our understanding of global carbon cycle on glacial-interglacial timescales. Here we report a new surface seawater pCO2 reconstruction for the western equatorial Pacific Ocean based on boron isotope analysis-a seawater pCO2 proxy-using two species of near-surface dwelling foraminifera from the same marine sediment core. The results indicate that the region remained a modest CO2 sink throughout the last deglaciation.

The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle

From 1.25 million to 700,000 years ago, the ice age cycle deepened and lengthened from 41,000- to 100,000-year periodicity, a transition that remains unexplained. Using surface- and bottom-dwelling foraminifera from the Antarctic Zone of the Southern Ocean to reconstruct the deep-to-surface supply of water during the ice ages of the past 1.5 million years, we found that a reduction in deep water supply and a concomitant freshening of the surface ocean coincided with the emergence of the high-amplitude 100,000-year glacial cycle. We propose that this slowing of deep-to-surface circulation (i.e., a longer residence time for Antarctic surface waters) prolonged ice ages by allowing the Antarctic halocline to strengthen, which increased the resistance of the Antarctic upper water column to orbitally paced drivers of carbon dioxide release.

Earth's radiative imbalance from the Last Glacial Maximum to the present

The energy imbalance at the top of the atmosphere determines the temporal evolution of the global climate, and vice versa changes in the climate system can alter the planetary energy fluxes. This interplay is fundamental to our understanding of Earth's heat budget and the climate system. However, even today, the direct measurement of global radiative fluxes is difficult, such that most assessments are based on changes in the total energy content of the climate system. We apply the same approach to estimate the long-term evolution of Earth's radiative imbalance in the past. New measurements of noble gas-derived mean ocean temperature from the European Project for Ice Coring in Antarctica Dome C ice core covering the last 40,000 y, combined with recent results from the West Antarctic Ice Sheet Divide ice core and the sea-level record, allow us to quantitatively reconstruct the history of the climate system energy budget. The temporal derivative of this quantity must be equal to the planetary radiative imbalance. During the deglaciation, a positive imbalance of typically +0.2 W⋅m^-2 is maintained for ∼10,000 y, however, with two distinct peaks that reach up to 0.4 W⋅m^-2 during times of substantially reduced Atlantic Meridional Overturning Circulation. We conclude that these peaks are related to net changes in ocean heat uptake, likely due to rapid changes in North Atlantic deep-water formation and their impact on the global radiative balance, while changes in cloud coverage, albeit uncertain, may also factor into the picture.

Erosion and deposition beneath the Subantarctic Front since the Early Oligocene

The Antarctic Circumpolar Current (ACC) spills across the Falkland Plateau into the South Atlantic as a series of high-velocity jets. These currents are a driving force for global overturning circulation, and affect climate by modulating CO₂ exchange between the atmosphere and ocean, but their timing of onset remains controversial. We present new evidence of strong currents associated with the Subantarctic Front (SAF) jet since the earliest Oligocene (~34 Ma) based on a widespread erosional surface on the Falkland Plateau, preserved below a 30,000 km² contourite sand deposit. This is the largest such feature ever to be recognized, and provides the most robust constraint of the initiation of the SAF to date. By contrast, the South Falkland Slope Drift is dominated by contourite mud of Pleistocene-Recent age, substantially younger than previous estimates, indicating a significant decrease in long-term current strength at that time. As ACC strength is primarily a function of the position of the South-Westerly Winds, our data indicates that associated currents are likely to increase substantially in a warming world. Likely implications include increased upwelling and associated carbon flux from the deep ocean to the atmosphere, a positive feedback loop not included in most future projections of atmospheric CO₂.

Aquifer systems extending far offshore on the U.S. Atlantic margin

Low-salinity submarine groundwater contained within continental shelves is a global phenomenon. Mechanisms for emplacing offshore groundwater include glacial processes that drove water into exposed continental shelves during sea-level low stands and active connections to onshore hydrologic systems. While low-salinity groundwater is thought to be abundant, its distribution and volume worldwide is poorly understood due to the limited number of observations. Here we image laterally continuous aquifers extending 90 km offshore New Jersey and Martha's Vineyard, Massachusetts, on the U.S. Atlantic margin using new shallow water electromagnetic geophysical methods. Our data provide more continuous constraints on offshore groundwater than previous models and present evidence for a connection between the modern onshore hydrologic system and offshore aquifers. We identify clinoforms as a previously unknown structural control on the lateral extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our data suggest a continuous submarine aquifer system spans at least 350 km of the U.S. Atlantic coast and contains about 2800 km³ of low-salinity groundwater. Our findings can be used to improve models of past glacial, eustatic, tectonic, and geomorphic processes on continental shelves and provide insight into shelf geochemistry, biogeochemical cycles, and the deep biosphere.

Neodymium isotope evidence for glacial-interglacial variability of deepwater transit time in the Pacific Ocean

There is evidence for greater carbon storage in the glacial deep Pacific, but it is uncertain whether it was caused by changes in ventilation, circulation, and biological productivity. The spatial ε_Nd evolution in the deep Pacific provides information on the deepwater transit time. Seven new foraminiferal ε_Nd records are presented to systematically constrain glacial to interglacial changes in deep Pacific overturning and two different ε_Nd evolution regimes occur spatially in the Pacific with reduced meridional ε_Nd gradients in glacials, suggesting a faster deep Pacific overturning circulation. This implies that greater glacial carbon storage due to sluggish circulation, that is believed to have occurred in the deep Atlantic, did not operate in a similar manner in the Pacific Ocean. Other mechanisms such as increased biological pump efficiency and poor high latitude air-sea exchange could be responsible for increased carbon storage in the glacial Pacific.

The response of deep Pacific overturning to glacial-interglacial climate change is still debated. Here the authors show a generally faster deep Pacific overturning operated in recent glacial periods based on a novel application of Nd isotopes recorded in foraminifera.

High-Resolution Benthic Mg/Ca Temperature Record of the Intermediate Water in the Denmark Strait Across D-O Stadial-Interstadial Cycles

Dansgaard-Oeschger (D-O) climate instabilities that took place during Marine Isotope Stage 3 are connected to changes in ocean circulation patterns and sea ice cover. Here we explore in detail the configuration of the water column of the Denmark Strait during D-O events 8-5. How the ocean currents and water masses within the Denmark Strait region responded and were connected to the North Atlantic are discussed. We investigate sediment core GS15-198-36CC, from the northern side of the Greenland-Iceland Ridge, at 30-year temporal resolution. Stable carbon and oxygen isotope reconstructions based on benthic foraminifera, together with a high-resolution benthic foraminiferal record of Mg/Ca paleothermometry, is presented. The site was bathed by warm intermediate waters during stadials and cool but gradually warming intermediate water during interstadials. We suggest that stadial conditions in the Denmark Strait are characterized by a well-stratified water column with a warm intermediate water mass that lies beneath a cold fresh body of water where sea ice and brine rejection work in consort to uphold the halocline conditions. Interstadial periods are not a pure replicate of modern times, but rather have two modes of operation, one similar to today, and the other incorporating a brief period of warm intermediate water and increased ventilation.

CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales

The cause of changes in atmospheric carbon dioxide (CO₂) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial-interglacial CO₂ change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere¹. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region². However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH-and thus the CO₂ chemistry-of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean's lower overturning cell, we find a close relationship between ocean pH and atmospheric CO₂: during intervals of low CO₂, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO₂, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO₂, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO₂ change, and show that deep-ocean CO₂ release can occur as a dynamic feedback to rapid climate change on centennial timescales.

Glacial expansion of oxygen-depleted seawater in the eastern tropical Pacific

Increased storage of carbon in the oceans has been proposed as a mechanism to explain lower concentrations of atmospheric carbon dioxide during ice ages; however, unequivocal signatures of this storage have not been found¹. In seawater, the dissolved gases oxygen and carbon dioxide are linked via the production and decay of organic material, with reconstructions of low oxygen concentrations in the past indicating an increase in biologically mediated carbon storage. Marine sediment proxy records have suggested that oxygen concentrations in the deep ocean were indeed lower during the last ice age, but that near-surface and intermediate waters of the Pacific Ocean-a large fraction of which are poorly oxygenated at present-were generally better oxygenated during the glacial^1-3. This vertical opposition could suggest a minimal net basin-integrated change in carbon storage. Here we apply a dual-proxy approach, incorporating qualitative upper-water-column and quantitative bottom-water oxygen reconstructions^4,5, to constrain changes in the vertical extent of low-oxygen waters in the eastern tropical Pacific since the last ice age. Our tandem proxy reconstructions provide evidence of a downward expansion of oxygen depletion in the eastern Pacific during the last glacial, with no indication of greater oxygenation in the upper reaches of the water column. We extrapolate our quantitative deep-water oxygen reconstructions to show that the respired carbon reservoir of the glacial Pacific was substantially increased, establishing it as an important component of the coupled mechanism that led to low levels of atmospheric carbon dioxide during the glacial.

Enhanced ocean-atmosphere carbon partitioning via the carbonate counter pump during the last deglacial

Several synergistic mechanisms were likely involved in the last deglacial atmospheric pCO₂ rise. Leading hypotheses invoke a release of deep-ocean carbon through enhanced convection in the Southern Ocean (SO) and concomitant decreased efficiency of the global soft-tissue pump (STP). However, the temporal evolution of both the STP and the carbonate counter pump (CCP) remains unclear, thus preventing the evaluation of their contributions to the pCO₂ rise. Here we present sedimentary coccolith records combined with export production reconstructions from the Subantarctic Pacific to document the leverage the SO biological carbon pump (BCP) has imposed on deglacial pCO₂. Our data suggest a weakening of BCP during the phases of carbon outgassing, due in part to an increased CCP along with higher surface ocean fertility and elevated [CO_2aq]. We propose that reduced BCP efficiency combined with enhanced SO ventilation played a major role in propelling the Earth out of the last ice age.

The contribution of the carbonate counter pump (CCP) to the last deglacial atmospheric CO₂ rise has yet been largely ignored. Here, the authors show that an increased CCP in the Subantarctic Pacific along with high surface ocean fertility and [CO_2aq], contributed in propelling the Earth out of the last ice age.

Spatio-temporal variability of processes across Antarctic ice-bed-ocean interfaces

Understanding how the Antarctic ice sheet will respond to global warming relies on knowledge of how it has behaved in the past. The use of numerical models, the only means to quantitatively predict the future, is hindered by limitations to topographic data both now and in the past, and in knowledge of how subsurface oceanic, glaciological and hydrological processes interact. Incorporating the variety and interplay of such processes, operating at multiple spatio-temporal scales, is critical to modeling the Antarctic's system evolution and requires direct observations in challenging locations. As these processes do not observe disciplinary boundaries neither should our future research.

Breakup of last glacial deep stratification in the South Pacific

Stratification of the deep Southern Ocean during the Last Glacial Maximum is thought to have facilitated carbon storage and subsequent release during the deglaciation as stratification broke down, contributing to atmospheric CO₂ rise. Here, we present neodymium isotope evidence from deep to abyssal waters in the South Pacific that confirms stratification of the deepwater column during the Last Glacial Maximum. The results indicate a glacial northward expansion of Ross Sea Bottom Water and a Southern Hemisphere climate trigger for the deglacial breakup of deep stratification. It highlights the important role of abyssal waters in sustaining a deep glacial carbon reservoir and Southern Hemisphere climate change as a prerequisite for the destabilization of the water column and hence the deglacial release of sequestered CO₂ through upwelling.

Ocean (de)oxygenation from the Last Glacial Maximum to the twenty-first century: insights from Earth System models

All Earth System models project a consistent decrease in the oxygen content of oceans for the coming decades because of ocean warming, reduced ventilation and increased stratification. But large uncertainties for these future projections of ocean deoxygenation remain for the subsurface tropical oceans where the major oxygen minimum zones are located. Here, we combine global warming projections, model-based estimates of natural short-term variability, as well as data and model estimates of the Last Glacial Maximum (LGM) ocean oxygenation to gain some insights into the major mechanisms of oxygenation changes across these different time scales. We show that the primary uncertainty on future ocean deoxygenation in the subsurface tropical oceans is in fact controlled by a robust compensation between decreasing oxygen saturation (O_2sat) due to warming and decreasing apparent oxygen utilization (AOU) due to increased ventilation of the corresponding water masses. Modelled short-term natural variability in subsurface oxygen levels also reveals a compensation between O_2sat and AOU, controlled by the latter. Finally, using a model simulation of the LGM, reproducing data-based reconstructions of past ocean (de)oxygenation, we show that the deoxygenation trend of the subsurface ocean during deglaciation was controlled by a combination of warming-induced decreasing O_2sat and increasing AOU driven by a reduced ventilation of tropical subsurface waters.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Synchronous precipitation reduction in the American Tropics associated with Heinrich 2

During the last ice age temperature in the North Atlantic oscillated in cycles known as Dansgaard-Oeschger (D-O) events. The magnitude of Caribbean hydroclimate change associated with D-O variability and particularly with stadial intervals, remains poorly constrained by paleoclimate records. We present a 3.3 thousand-year long stalagmite δ¹⁸O record from the Yucatan Peninsula (YP) that spans the interval between 26.5 and 23.2 thousand years before present. We estimate quantitative precipitation variability and the high resolution and dating accuracy of this record allow us to investigate how rainfall in the region responds to D-O events. Quantitative precipitation estimates are based on observed regional amount effect variability, last glacial paleotemperature records, and estimates of the last glacial oxygen isotopic composition of precipitation based on global circulation models (GCMs). The new precipitation record suggests significant low latitude hydrological responses to internal modes of climate variability and supports a role of Caribbean hydroclimate in helping Atlantic Meridional Overturning Circulation recovery during D-O events. Significant in-phase precipitation reduction across the equator in the tropical Americas associated with Heinrich event 2 is suggested by available speleothem oxygen isotope records.

Spatiotemporal variability in the δ18O-salinity relationship of seawater across the tropical Pacific Ocean

The relationship between salinity and the stable oxygen isotope ratio of seawater (δ¹⁸O_sw) is of utmost importance to the quantitative reconstruction of past changes in salinity from δ¹⁸O values of marine carbonates. This relationship is often considered to be uniform across water masses, but the constancy of the δ¹⁸O_sw-salinity relationship across space and time remains uncertain, as δ¹⁸O_sw responds to varying atmospheric vapor sources and pathways, while salinity does not. Here we present new δ¹⁸O_sw-salinity data from sites spanning the tropical Pacific Ocean. New data from Palau, Papua New Guinea, Kiritimati, and Galápagos show slopes ranging from 0.09 ‰/psu in the Galápagos to 0.32‰/psu in Palau. The slope of the δ¹⁸O_sw-salinity relationship is higher in the western tropical Pacific versus the eastern tropical Pacific in observations and in two isotope-enabled climate models. A comparison of δ¹⁸O_sw-salinity relationships derived from short-term spatial surveys and multi-year time series at Papua New Guinea and Galápagos suggests spatial relationships can be substituted for temporal relationships at these sites, at least within the time period of the investigation. However, the δ¹⁸O_sw-salinity relationship varied temporally at Palau, likely in response to water mass changes associated with interannual El Niño-Southern Oscillation (ENSO) variability, suggesting nonstationarity in this local δ¹⁸O_sw-salinity relationship. Applying local δ¹⁸O_sw-salinity relationships in a coral δ¹⁸O forward model shows that using a constant, basin-wide δ¹⁸O_sw-salinity slope can both overestimate and underestimate the contribution of δ¹⁸O_sw to carbonate δ¹⁸O variance at individual sites in the western tropical Pacific.

Porewater salinity reveals past lake-level changes in Lake Van, the Earth's largest soda lake

In closed-basin lakes, sediment porewater salinity can potentially be used as a conservative tracer to reconstruct past fluctuations in lake level. However, until now, porewater salinity profiles did not allow quantitative estimates of past lake-level changes because, in contrast to the oceans, significant salinity changes (e.g., local concentration minima and maxima) had never been observed in lacustrine sediments. Here we show that the salinity measured in the sediment pore water of Lake Van (Turkey) allows straightforward reconstruction of two major transgressions and a major regression that occurred during the last 250 ka. We observed strong changes in the vertical salinity profiles of the pore water of the uppermost 100 m of the sediments in Lake Van. As the salinity balance of Lake Van is almost at steady-state, these salinity changes indicate major lake-level changes in the past. In line with previous studies on lake terraces and with seismic and sedimentological surveys, we identify two major transgressions of up to +105 m with respect to the current lake level at about 135 ka BP and 248 ka BP starting at the onset of the two previous interglacials (MIS5e and MIS7), and a major regression of about −200 m at about 30 ka BP during the last ice age.

Timing and nature of AMOC recovery across Termination 2 and magnitude of deglacial CO2 change

Large amplitude variations in atmospheric CO₂ were associated with glacial terminations of the Late Pleistocene. Here we provide multiple lines of evidence suggesting that the ∼20 p.p.m.v. overshoot in CO₂ at the end of Termination 2 (T2) ∼129 ka was associated with an abrupt (≤400 year) deepening of Atlantic Meridional Overturning Circulation (AMOC). In contrast to Termination 1 (T1), which was interrupted by the Bølling-Allerød (B-A), AMOC recovery did not occur until the very end of T2, and was characterized by pronounced formation of deep waters in the NW Atlantic. Considering the variable influences of ocean circulation change on atmospheric CO₂, we suggest that the net change in CO₂ across the last 2 terminations was approximately equal if the transient effects of deglacial oscillations in ocean circulation are taken into account.

Differences in the sequence and timing of ocean circulation changes across glacial terminations could affect the magnitude of deglacial atmospheric CO₂ rise. Here, the authors argue that late ocean circulation recovery during the penultimate deglaciation (T2) led to a larger rise in CO₂ compared with T1.

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Earth's climate has undergone dramatic shifts between glacial and interglacial time periods, with high-latitude temperature changes on the order of 5-10 °C. These climatic shifts have been associated with major rearrangements in the deep ocean circulation and stratification, which have likely played an important role in the observed atmospheric carbon dioxide swings by affecting the partitioning of carbon between the atmosphere and the ocean. The mechanisms by which the deep ocean circulation changed, however, are still unclear and represent a major challenge to our understanding of glacial climates. This study shows that various inferred changes in the deep ocean circulation and stratification between glacial and interglacial climates can be interpreted as a direct consequence of atmospheric temperature differences. Colder atmospheric temperatures lead to increased sea ice cover and formation rate around Antarctica. The associated enhanced brine rejection leads to a strongly increased deep ocean stratification, consistent with high abyssal salinities inferred for the last glacial maximum. The increased stratification goes together with a weakening and shoaling of the interhemispheric overturning circulation, again consistent with proxy evidence for the last glacial. The shallower interhemispheric overturning circulation makes room for slowly moving water of Antarctic origin, which explains the observed middepth radiocarbon age maximum and may play an important role in ocean carbon storage.

Alkali-earth metal bridges formed in biofilm matrices regulate the uptake of fluoroquinolone antibiotics and protect against bacterial apoptosis

Bacterially extracellular biofilms play a critical role in relieving toxicity of fluoroquinolone antibiotic (FQA) pollutants, yet it is unclear whether antibiotic attack may be defused by a bacterial one-two punch strategy associated with metal-reinforced detoxification efficiency. Our findings help to assign functions to specific structural features of biofilms, as they strongly imply a molecularly regulated mechanism by which freely accessed alkali-earth metals in natural waters affect the cellular uptake of FQAs at the water-biofilm interface. Specifically, formation of alkali-earth-metal (Ca²⁺ or Mg²⁺) bridge between modeling ciprofloxacin and biofilms of Escherichia coli regulates the trans-biofilm transport rate of FQAs towards cells (135-nm-thick biofilm). As the addition of Ca²⁺ and Mg²⁺ (0-3.5 mmol/L, CIP: 1.25 μmol/L), the transport rates were reduced to 52.4% and 63.0%, respectively. Computational chemistry analysis further demonstrated a deprotonated carboxyl in the tryptophan residues of biofilms acted as a major bridge site, of which one side is a metal and the other is a metal girder jointly connected to the carboxyl and carbonyl of a FQA. The bacterial growth rate depends on the bridging energy at anchoring site, which underlines the environmental importance of metal bridge formed in biofilm matrices in bacterially antibiotic resistance.

Numerical investigation of initial condition effects on Rayleigh-Taylor instability with acceleration reversals

The influence of initial conditions on miscible incompressible baroclinically driven Rayleigh-Taylor instability undergoing nonuniform acceleration is explored computationally using an implicit large eddy simulation (ILES) technique. We consider the particular case of evolution during multiple reversals of acceleration direction, where the flow is alternately statically stable or unstable. In the unstable phase, the flow is driven by the baroclinic release of potential energy, whereas in the stable phase, work is done against the density stratification with the energy exchange taking place by wavelike mechanisms. These dynamics are fundamentally different; here, we track the evolution of volume-averaged turbulent statistics that are most sensitive to changes in the distribution of spectral power and bandwidth of the initial conditions as the flow alternates between dynamical regimes due to acceleration reversal.

Perspectives on heterococcolith geochemical proxies based on high-resolution X-ray fluorescence mapping

Heterococcoliths are micron-scale calcite platelets produced by coccolithophores. They have been the most abundant and continuous fossil record over the last 215 million years (Myr), offering great potential for geochemical studies, although the heterococcolith fossil record remains underutilised in this domain. We have mapped heterococcoliths' composition using X-ray fluorescence (XRF) with a 100-nm resolution beam to decipher element distributions in heterococcoliths and to investigate the potential development of geochemical proxies for palaeoceanography. The study presents two Middle Jurassic Watznaueria britannica heterococcoliths from Cabo Mondego, Portugal. XRF analysis was performed with a 17 keV incident energy beam at the European Synchrotron Radiation Facility ID22NI beamline to study elements from Sr down to S. Ca, Sr and Mn are distributed following the heterococcolith crystalline arrangement. Cl, Br and S display an homogeneous distribution, whereas K, Fe, Cu, Zn and Rb are concentrated at the edges and in the central area of the heterococcoliths. Distributions of K, Fe, Ti, Fe, Cu, Zn, Rb and to a lesser extent V and Cr are highly influenced by clay contamination and peripheral diagenetic overgrowth. Mn is related to diagenetic Mn-rich CaCO3 overgrowth on top of or between heterococcoliths shields. Cl and Br are likely to be present in heterococcoliths inside interstitial nano-domains. We assume that the cytoplasm [Cl(-) ] and [Br(-) ] are mediated and constant during heterococcolithogenesis. Assuming a linear correlation between cytoplasm [Cl(-) ] and sea water [Cl(-) ], heterococcolith Cl may have potential as a salinity proxy. As S is incorporated into heterococcoliths by sulphated polysaccharides, our study suggests a role for such polysaccharides in heterococcolithogenesis for at least 170 Myr. The low Sr/Ca in the W. britannica specimens studied here may either highlight an unusual cellular physiology of Mesozoic coccolithophores or result from low growth rates in oligotrophic environments.

Biological and physical controls in the Southern Ocean on past millennial-scale atmospheric CO2 changes

Millennial-scale climate changes during the last glacial period and deglaciation were accompanied by rapid changes in atmospheric CO2 that remain unexplained. While the role of the Southern Ocean as a 'control valve' on ocean-atmosphere CO2 exchange has been emphasized, the exact nature of this role, in particular the relative contributions of physical (for example, ocean dynamics and air-sea gas exchange) versus biological processes (for example, export productivity), remains poorly constrained. Here we combine reconstructions of bottom-water [O2], export production and (14)C ventilation ages in the sub-Antarctic Atlantic, and show that atmospheric CO2 pulses during the last glacial- and deglacial periods were consistently accompanied by decreases in the biological export of carbon and increases in deep-ocean ventilation via southern-sourced water masses. These findings demonstrate how the Southern Ocean's 'organic carbon pump' has exerted a tight control on atmospheric CO2, and thus global climate, specifically via a synergy of both physical and biological processes.

Evolution of South Atlantic density and chemical stratification across the last deglaciation

Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ(18)O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ(13)C and foraminifer/coral (14)C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.

Combining marine macroecology and palaeoecology in understanding biodiversity: microfossils as a model

There is growing interest in the integration of macroecology and palaeoecology towards a better understanding of past, present, and anticipated future biodiversity dynamics. However, the empirical basis for this integration has thus far been limited. Here we review prospects for a macroecology-palaeoecology integration in biodiversity analyses with a focus on marine microfossils [i.e. small (or small parts of) organisms with high fossilization potential, such as foraminifera, ostracodes, diatoms, radiolaria, coccolithophores, dinoflagellates, and ichthyoliths]. Marine microfossils represent a useful model system for such integrative research because of their high abundance, large spatiotemporal coverage, and good taxonomic and temporal resolution. The microfossil record allows for quantitative cross-scale research designs, which help in answering fundamental questions about marine biodiversity, including the causes behind similarities in patterns of latitudinal and longitudinal variation across taxa, the degree of constancy of observed gradients over time, and the relative importance of hypothesized drivers that may explain past or present biodiversity patterns. The inclusion of a deep-time perspective based on high-resolution microfossil records may be an important step for the further maturation of macroecology. An improved integration of macroecology and palaeoecology would aid in our understanding of the balance of ecological and evolutionary mechanisms that have shaped the biosphere we inhabit today and affect how it may change in the future.

The seasonal sea-ice zone in the glacial Southern Ocean as a carbon sink

Reduced surface-deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2. However, identification of the biological and physical conditions involved and the related processes remains incomplete. Here we specify Southern Ocean surface-subsurface contrasts using a new tool, the combined oxygen and silicon isotope measurement of diatom and radiolarian opal, in combination with numerical simulations. Our data do not indicate a permanent glacial halocline related to melt water from icebergs. Corroborated by numerical simulations, we find that glacial surface stratification was variable and linked to seasonal sea-ice changes. During glacial spring-summer, the mixed layer was relatively shallow, while deeper mixing occurred during fall-winter, allowing for surface-ocean refueling with nutrients from the deep reservoir, which was potentially richer in nutrients than today. This generated specific carbon and opal export regimes turning the glacial seasonal sea-ice zone into a carbon sink.

Antarctic sea ice control on ocean circulation in present and glacial climates

In the modern climate, the ocean below 2 km is mainly filled by waters sinking into the abyss around Antarctica and in the North Atlantic. Paleoproxies indicate that waters of North Atlantic origin were instead absent below 2 km at the Last Glacial Maximum, resulting in an expansion of the volume occupied by Antarctic origin waters. In this study we show that this rearrangement of deep water masses is dynamically linked to the expansion of summer sea ice around Antarctica. A simple theory further suggests that these deep waters only came to the surface under sea ice, which insulated them from atmospheric forcing, and were weakly mixed with overlying waters, thus being able to store carbon for long times. This unappreciated link between the expansion of sea ice and the appearance of a voluminous and insulated water mass may help quantify the ocean's role in regulating atmospheric carbon dioxide on glacial-interglacial timescales. Previous studies pointed to many independent changes in ocean physics to account for the observed swings in atmospheric carbon dioxide. Here it is shown that many of these changes are dynamically linked and therefore must co-occur.

Sea-level and deep-sea-temperature variability over the past 5.3 million years

Ice volume (and hence sea level) and deep-sea temperature are key measures of global climate change. Sea level has been documented using several independent methods over the past 0.5 million years (Myr). Older periods, however, lack such independent validation; all existing records are related to deep-sea oxygen isotope (δ(18)O) data that are influenced by processes unrelated to sea level. For deep-sea temperature, only one continuous high-resolution (Mg/Ca-based) record exists, with related sea-level estimates, spanning the past 1.5 Myr. Here we present a novel sea-level reconstruction, with associated estimates of deep-sea temperature, which independently validates the previous 0-1.5 Myr reconstruction and extends it back to 5.3 Myr ago. We find that deep-sea temperature and sea level generally decreased through time, but distinctly out of synchrony, which is remarkable given the importance of ice-albedo feedbacks on the radiative forcing of climate. In particular, we observe a large temporal offset during the onset of Plio-Pleistocene ice ages, between a marked cooling step at 2.73 Myr ago and the first major glaciation at 2.15 Myr ago. Last, we tentatively infer that ice sheets may have grown largest during glacials with more modest reductions in deep-sea temperature.

Offshore fresh groundwater reserves as a global phenomenon

The flow of terrestrial groundwater to the sea is an important natural component of the hydrological cycle. This process, however, does not explain the large volumes of low-salinity groundwater that are found below continental shelves. There is mounting evidence for the global occurrence of offshore fresh and brackish groundwater reserves. The potential use of these non-renewable reserves as a freshwater resource provides a clear incentive for future research. But the scope for continental shelf hydrogeology is broader and we envisage that it can contribute to the advancement of other scientific disciplines, in particular sedimentology and marine geochemistry.