Increased biological productivity and export production in the glacial Southern Ocean

A millimeter-scale insight into formation mechanism of lacustrine black shale in tephra deposition background

To reveal the role of tephra in the deposition of black shale during periods of volcanic activity, we performed lithostratigraphic and geochemical analyses on 14 horizontally sliced samples drilled from a 2-cm-thick black shale interval in the lower Ch7 Member of the Upper Triassic Yanchang Formation, southern Ordos Basin. Results indicate that fewer plankton is preserved during tephra deposition than during periods of volcanic quiescence. With the decrease of volcanic activities and tephra deposition, the abundance of redox-sensitive trace elements (RSTEs) and biolimiting elements increases, while terrigenous elements gradually decrease, resulting in the improvement of organic matter (OM) preservation. Paleoenvironmental proxies suggest that the climate during the Late Triassic was generally warm and humid. However, subsequent intense volcanic eruptions may have caused climatic cooling that affected the water column, resulting in enhanced salinity, primary production, water stratification, and bottom water anoxia, leading to enhanced organic carbon production and preservation. Primary productivity and redox conditions controlled the accumulation of organic carbon. Although physical and chemical reactions relating to the deposition of tephra into water are short-lived, climate change induced by volcanic eruptions is the critical cause of black shale formation.

Antiphased dust deposition and productivity in the Antarctic Zone over 1.5 million years

The Southern Ocean paleoceanography provides key insights into how iron fertilization and oceanic productivity developed through Pleistocene ice-ages and their role in influencing the carbon cycle. We report a high-resolution record of dust deposition and ocean productivity for the Antarctic Zone, close to the main dust source, Patagonia. Our deep-ocean records cover the last 1.5 Ma, thus doubling that from Antarctic ice-cores. We find a 5 to 15-fold increase in dust deposition during glacials and a 2 to 5-fold increase in biogenic silica deposition, reflecting higher ocean productivity during interglacials. This antiphasing persisted throughout the last 25 glacial cycles. Dust deposition became more pronounced across the Mid-Pleistocene Transition (MPT) in the Southern Hemisphere, with an abrupt shift suggesting more severe glaciations since ~0.9 Ma. Productivity was intermediate pre-MPT, lowest during the MPT and highest since 0.4 Ma. Generally, glacials experienced extended sea-ice cover, reduced bottom-water export and Weddell Gyre dynamics, which helped lower atmospheric CO₂ levels.

10Be depositional flux variation in the central Indian Ocean during the last 43 ka

The advent of Accelerator Mass Spectrometer (AMS) enhanced the application of meteoric ¹⁰Be (half-life of 1.39 Ma) as a tracer for understanding earth surface processes on thousand to million-year time scales. However, for the majority of applications, an adequate understanding of the ¹⁰Be depositional flux is a prerequisite. A number of efforts have been made to understand both spatial and temporal variation of ¹⁰Be depositional flux. Yet, due to the limited globally distributed dataset and modulation of the ¹⁰Be signal by local processes, a significant offset is observed between model-derived and measured deposition rates of ¹⁰Be. In this study, an attempt has been made to determine the ¹⁰Be depositional flux from a marine sediment core from the central Indian Ocean chronologically constrained with the AMS radiocarbon dating and ¹⁰Be concentration measured with AMS. The ¹⁰Be depositional flux estimates using weak leaching method are found to be nearly 44% lower compared to the strong leaching method. The calculated ¹⁰Be depositional flux during the Holocene varies between 9.63 and 13.01 × 10⁵ atoms/cm²/yr, which is 2-28% lower compared to the modeled depositional flux for the region. The difference observed in ¹⁰Be depositional flux could be due to the local processes (such as boundary scavenging, changing rate of sediment deposition at the location) affecting ¹⁰Be deposition into the sediment column or offset associated with the model estimations. The changes in ¹⁰Be depositional flux and the ¹⁰Be/⁹Be ratio have been reconstructed up to 43 ka. An increase in the ¹⁰Be/⁹Be ratio during 28 to 43 ka is observed due to the lower geomagnetic field intensity during the period. A high-resolution ¹⁰Be/⁹Be ratio reconstruction shows a peak at 41.2 ka, which can be attributed to the Laschamp event.

Persistent deep water anoxia in the eastern South Atlantic during the last ice age

During the last glacial interval, marine sediments recorded reduced current ventilation within the ocean interior below water depths of approximately >1,500 m [B. A. Hoogakker et al., Nat. Geosci. 8, 40-43 (2015)]. The degree of the associated oxygen depletion in the different ocean basins, however, is still poorly constrained. Here, we present sedimentary records of redox-sensitive metals from the southwest African margin. These records show evidence of continuous bottom water anoxia in the eastern South Atlantic during the last glaciation that led to enhanced carbon burial over a prolonged period of time. Our geochemical data indicate that upwelling-related productivity and the associated oxygen minimum zone in the eastern South Atlantic shifted far seaward during the last glacial period and only slowly retreated during deglaciation times. While increased productivity during the last ice age may have contributed to oxygen depletion in bottom waters, especially on the upper slope, slow-down of the Late Quaternary deep water circulation pattern [Rutberg et al., Nature 405, 935-938 (2000)] appears to be the ultimate driver of anoxic conditions in deep waters.

Southern Ocean upwelling, Earth's obliquity, and glacial-interglacial atmospheric CO2 change

Previous studies have suggested that during the late Pleistocene ice ages, surface-deep exchange was somehow weakened in the Southern Ocean's Antarctic Zone, which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages. Here, high-resolution diatom-bound nitrogen isotope measurements from the Indian sector of the Antarctic Zone reveal three modes of change in Southern Westerly Wind-driven upwelling, each affecting atmospheric carbon dioxide. Two modes, related to global climate and the bipolar seesaw, have been proposed previously. The third mode-which arises from the meridional temperature gradient as affected by Earth's obliquity (axial tilt)-can explain the lag of atmospheric carbon dioxide behind climate during glacial inception and deglaciation. This obliquity-induced lag, in turn, makes carbon dioxide a delayed climate amplifier in the late Pleistocene glacial cycles.

Million-year lag times in a post-orogenic sediment conveyor

Understanding how sediment transport and storage will delay, attenuate, and even erase the erosional signal of tectonic and climatic forcings has bearing on our ability to read and interpret the geologic record effectively. Here, we estimate sediment transit times in Australia's largest river system, the Murray-Darling basin, by measuring downstream changes in cosmogenic 26Al/10Be/14C ratios in modern river sediment. Results show that the sediments have experienced multiple episodes of burial and reexposure, with cumulative lag times exceeding 1 Ma in the downstream reaches of the Murray and Darling rivers. Combined with low sediment supply rates and old sediment blanketing the landscape, we posit that sediment recycling in the Murray-Darling is an important and ongoing process that will substantially delay and alter signals of external environmental forcing transmitted from the sediment's hinterland.

Deep Equatorial Pacific Ocean Oxygenation and Atmospheric CO2 Over The Last Ice Age

Ventilation of carbon stored in the deep ocean is thought to play an important role in atmospheric CO₂ increases associated with Pleistocene deglaciations. The presence of this respired carbon has been recorded by an array of paleoceanographic proxies from various locations across the global ocean. Here we present a new sediment core from the Eastern Equatorial Pacific (EEP) Ocean spanning the last 180,000 years and reconstruct high-resolution ²³⁰Th-derived fluxes of ²³²Th and excess barium, along with redox-sensitive uranium concentrations to examine past variations in dust delivery, export productivity, and bottom-water oxygenation, respectively. Our bottom-water oxygenation record is compared to other similar high-resolution records from across the Pacific and in the Southern Ocean. We suggest that the deep Pacific is a site of respired carbon storage associated with periods of decreased global atmospheric CO₂ concentration during the LGM, confirming the conclusions from a wealth of previous studies. However, our study is the first to show a similar relationship beyond the last glacial, extending to at least 70,000 years.

Ice sheets matter for the global carbon cycle

The cycling of carbon on Earth exerts a fundamental influence upon the greenhouse gas content of the atmosphere, and hence global climate over millennia. Until recently, ice sheets were viewed as inert components of this cycle and largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon (>104 Pg C) and nutrients. Here we assess the active role of ice sheets in the global carbon cycle and potential ramifications of enhanced melt and ice discharge in a warming world.

Isotopes from fossil coronulid barnacle shells record evidence of migration in multiple Pleistocene whale populations

Migration is an integral feature of modern mysticete whale ecology, and the demands of migration may have played a key role in shaping mysticete evolutionary history. Constraining when migration became established and assessing how it has changed through time may yield valuable insight into the evolution of mysticete whales and the oceans in which they lived. However, there are currently few data which directly assess prehistoric mysticete migrations. Here we show that calcite δ¹⁸O profiles of two species of modern whale barnacles (coronulids) accurately reflect the known migration routes of their host whales. We then analyze well-preserved fossil coronulids from three different locations along the eastern Pacific coast, finding that δ¹⁸O profiles from these fossils exhibit trends and ranges similar to modern specimens. Our results demonstrate that migration is an ancient behavior within the humpback and gray whale lineages and that multiple Pleistocene populations were undertaking migrations of an extent similar to those of the present day.

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.

Reconstructing the Dust Cycle in Deep Time: the Case of the Late Paleozoic Icehouse

Atmospheric dust constitutes particles <100 μm, or deposits thereof (continental or marine); dust includes ‘loess,’ defined as continental aeolian silt (4–62.5 μm). Dust is well-known from Earth's near-time (mostly Quaternary) record, and recognized as a high-fidelity archive of climate, but remains under-recognized for deep time. Attributes such as thickness, grain size, magnetism, pedogenesis, and provenance of dust form valuable indicators of paleoclimate to constrain models of atmospheric dustiness. Additionally, dust acts as an agent of climate change via both direct and indirect effects on radiative forcing, and on productivity, and thus the biosphere and carbon cycling. Dust from the late Paleozoic of western equatorial Pangea reflects ultimate derivation from orogens (ancestral Rocky Mountains, Central Pangean Mountains), whereas dust from southwestern Pangea (Bolivia) reflects both proximal volcanism and crustal material. Records of dust conducive to cyclostratigraphic analysis, such as data on dust inputs from carbonate sections, or magnetism in paleo-loess, reveal dust cyclicity at Milankovitch timescales, but resolution is compromised if records are too brief, or irregular in interval or magnitude of the attribute being measured. Climate modeling enables identification of the primary regions of dust sourcing in deep time, and impacts of dust on radiative balance and biogeochemistry. Deep-time modeling remains preliminary, but is achievable, and indicates principal dust sources in the Pangean subtropics, with sources increasing during colder climates. Carbon cycle modeling suggests that glacial-phase dust increases stimulated extreme productivity, potentially increasing algal activity and perturbing ecosystem compositions of the late Paleozoic.

Quantifying trace element and isotope fluxes at the ocean-sediment boundary: a review

Quantifying fluxes of trace elements and their isotopes (TEIs) at the ocean's sediment-water boundary is a pre-eminent challenge to understand their role in the present, past and future ocean. There are multiple processes that drive the uptake and release of TEIs, and properties that determine their rates are unevenly distributed (e.g. sediment composition, redox conditions and (bio)physical dynamics). These factors complicate our efforts to find, measure and extrapolate TEI fluxes across ocean basins. GEOTRACES observations are unveiling the oceanic distributions of many TEIs for the first time. These data evidence the influence of the sediment-water boundary on many TEI cycles, and underline the fact that our knowledge of the source-sink fluxes that sustain oceanic distributions is largely missing. Present flux measurements provide low spatial coverage and only part of the empirical basis needed to predict TEI flux variations. Many of the advances and present challenges facing TEI flux measurements are linked to process studies that collect sediment cores, pore waters, sinking material or seawater in close contact with sediments. However, such sampling has not routinely been viable on GEOTRACES expeditions. In this article, we recommend approaches to address these issues: firstly, with an interrogation of emergent data using isotopic mass-balance and inverse modelling techniques; and secondly, by innovating pursuits of direct TEI flux measurements. We exemplify the value of GEOTRACES data with a new inverse model estimate of benthic Al flux in the North Atlantic Ocean. Furthermore, we review viable flux measurement techniques tailored to the sediment-water boundary. We propose that such activities are aimed at regions that intersect the GEOTRACES Science Plan on the basis of seven criteria that may influence TEI fluxes: sediment provenance, composition, organic carbon supply, redox conditions, sedimentation rate, bathymetry and the benthic nepheloid inventory.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Spatio-Temporal Monitoring and Ecological Significance of Retrievable Pelagic Heterotrophic Bacteria in Kongsfjorden, an Arctic Fjord

Kongsfjorden is a glacial fjord in the Arctic that is influenced by both Atlantic and Arctic water masses. In the present report retrievable heterotrophic bacteria isolated from two distinct zones (outer and inner fjord) of Kongsfjorden was studied during summer to fall of 2012. 16S rRNA gene sequences of the retrievable heterotrophic bacteria corresponded to γ-proteobacteria (13 phylotypes), α-proteobacteria (3 phylotypes), Bacteroidetes (4 phylotypes) and Actinobacteria (2 phylotypes). The heterotrophic bacterial community structure was fundamentally different in different months which could be linked to changes in the water masses and/or phytoplankton bloom dynamics. It is hypothesized that monitoring the retrievable heterotrophic bacterial assemblage in the fjord would give valuable insights into the complex ecological role they play under extreme and dynamic conditions.

231Pa/230Th evidence for a weakened but persistent Atlantic meridional overturning circulation during Heinrich Stadial 1

The strength of Atlantic meridional overturning circulation is believed to affect the climate over glacial-interglacial and millennial timescales. The marine sedimentary (231)Pa/(230)Th ratio is a promising paleocirculation proxy, but local particle effects may bias individual reconstructions. Here we present new Atlantic sedimentary (231)Pa/(230)Th data from the Holocene, the last glacial maximum and Heinrich Stadial 1, a period of abrupt cooling ca. 17,500 years ago. We combine our results with published data from these intervals to create a spatially distributed sedimentary (231)Pa/(230)Th database. The data reveal a net (231)Pa deficit during each period, consistent with persistent (231)Pa export. In highly resolved cores, Heinrich (231)Pa/(230)Th ratios exceed glacial ratios at nearly all depths, indicating a significant reduction, although not cessation, of overturning during Heinrich Stadial 1. These results support the inference that weakened overturning was a driver of Heinrich cooling, while suggesting that abrupt climate oscillations do not necessarily require a complete shutdown of overturning.

Biological response to millennial variability of dust and nutrient supply in the Subantarctic South Atlantic Ocean

Fluxes of lithogenic material and fluxes of three palaeo-productivity proxies (organic carbon, biogenic opal and alkenones) over the past 100,000 years were determined using the (230)Th-normalization method in three sediment cores from the Subantarctic South Atlantic Ocean. Features in the lithogenic flux record of each core correspond to similar features in the record of dust deposition in the EPICA Dome C ice core. Biogenic fluxes correlate with lithogenic fluxes in each sediment core. Our preferred interpretation is that South American dust, most probably from Patagonia, constitutes a major source of lithogenic material in Subantarctic South Atlantic sediments, and that past biological productivity in this region responded to variability in the supply of dust, probably due to biologically available iron carried by the dust. Greater nutrient supply as well as greater nutrient utilization (stimulated by dust) contributed to Subantarctic productivity during cold periods, in contrast to the region south of the Antarctic Polar Front (APF), where reduced nutrient supply during cold periods was the principal factor limiting productivity. The anti-phased patterns of productivity on opposite sides of the APF point to shifts in the physical supply of nutrients and to dust as cofactors regulating productivity in the Southern Ocean.

Iron fertilization of the Subantarctic ocean during the last ice age

John H. Martin, who discovered widespread iron limitation of ocean productivity, proposed that dust-borne iron fertilization of Southern Ocean phytoplankton caused the ice age reduction in atmospheric carbon dioxide (CO2). In a sediment core from the Subantarctic Atlantic, we measured foraminifera-bound nitrogen isotopes to reconstruct ice age nitrate consumption, burial fluxes of iron, and proxies for productivity. Peak glacial times and millennial cold events are characterized by increases in dust flux, productivity, and the degree of nitrate consumption; this combination is uniquely consistent with Subantarctic iron fertilization. The associated strengthening of the Southern Ocean's biological pump can explain the lowering of CO2 at the transition from mid-climate states to full ice age conditions as well as the millennial-scale CO2 oscillations.

Continuous flow analysis of labile iron in ice-cores

The important active and passive role of mineral dust aerosol in the climate and the global carbon cycle over the last glacial/interglacial cycles has been recognized. However, little data on the most important aeolian dust-derived biological micronutrient, iron (Fe), has so far been available from ice-cores from Greenland or Antarctica. Furthermore, Fe deposition reconstructions derived from the palaeoproxies particulate dust and calcium differ significantly from the Fe flux data available. The ability to measure high temporal resolution Fe data in polar ice-cores is crucial for the study of the timing and magnitude of relationships between geochemical events and biological responses in the open ocean. This work adapts an existing flow injection analysis (FIA) methodology for low-level trace Fe determinations with an existing glaciochemical analysis system, continuous flow analysis (CFA) of ice-cores. Fe-induced oxidation of N,N'-dimethyl-p-pheylenediamine (DPD) is used to quantify the biologically more important and easily leachable Fe fraction released in a controlled digestion step at pH ~1.0. The developed method was successfully applied to the determination of labile Fe in ice-core samples collected from the Antarctic Byrd ice-core and the Greenland Ice-Core Project (GRIP) ice-core.

Two modes of change in Southern Ocean productivity over the past million years

Export of organic carbon from surface waters of the Antarctic Zone of the Southern Ocean decreased during the last ice age, coinciding with declining atmospheric carbon dioxide (CO(2)) concentrations, signaling reduced exchange of CO(2) between the ocean interior and the atmosphere. In contrast, in the Subantarctic Zone, export production increased into ice ages coinciding with rising dust fluxes, thus suggesting iron fertilization of subantarctic phytoplankton. Here, a new high-resolution productivity record from the Antarctic Zone is compiled with parallel subantarctic data over the past million years. Together, they fit the view that the combination of these two modes of Southern Ocean change determines the temporal structure of the glacial-interglacial atmospheric CO(2) record, including during the interval of "lukewarm" interglacials between 450 and 800 thousand years ago.

Southern Ocean dust-climate coupling over the past four million years

Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p.p.m.v.) of the decrease (80-100 p.p.m.v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.

A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean

The seeding of an expanse of surface waters in the equatorial Pacific Ocean with low concentrations of dissolved iron triggered a massive phytoplankton bloom which consumed large quantities of carbon dioxide and nitrate that these microscopic plants cannot fully utilize under natural conditions. These and other observations provide unequivocal support for the hypothesis that phytoplankton growth in this oceanic region is limited by iron bioavailability.

Externalities of fugitive dust

It is known that fugitive dust can cause human health and environmental problems, alone or in combination with other air pollutants. These problems are referred to as 'external costs' that have been traditionally ignored. However, there is a growing interest towards quantifying externalities to assist policy and decision-making. With this in mind, the present study aimed at discussing the environmental regulations that deal with fugitive dust, the impact of fugitive dust on human health and global climate system, and the available methods for calculating fugitive dust externalities. The damage cost associated with human health and global environmental problems was predicted based on the environmental strategy priority model. The damage cost estimated by the model ranged from 40 to 374 EUR/kg of emitted fugitive dust with a mean value of 120 EUR/kg of emitted fugitive dust. It was also found that PM(2.5) and PM(10) have contributed to about 60% and 36% of the estimated damage cost, respectively. The remaining 4% was attributed to both nitrate and sulfate aerosols.

Glacial/Interglacial Changes in Subarctic North Pacific Stratification

Since the first evidence of low algal productivity during ice ages in the Antarctic Zone of the Southern Ocean was discovered, there has been debate as to whether it was associated with increased polar ocean stratification or with sea-ice cover, shortening the productive season. The sediment concentration of biogenic barium at Ocean Drilling Program site 882 indicates low algal productivity during ice ages in the Subarctic North Pacific as well. Site 882 is located southeast of the summer sea-ice extent even during glacial maxima, ruling out sea-ice-driven light limitation and supporting stratification as the explanation, with implications for the glacial cycles of atmospheric carbon dioxide concentration.

Getting to know you: reputation and trust in a two-person economic exchange

Using a multiround version of an economic exchange (trust game), we report that reciprocity expressed by one player strongly predicts future trust expressed by their partner-a behavioral finding mirrored by neural responses in the dorsal striatum. Here, analyses within and between brains revealed two signals-one encoded by response magnitude, and the other by response timing. Response magnitude correlated with the "intention to trust" on the next play of the game, and the peak of these "intention to trust" responses shifted its time of occurrence by 14 seconds as player reputations developed. This temporal transfer resembles a similar shift of reward prediction errors common to reinforcement learning models, but in the context of a social exchange. These data extend previous model-based functional magnetic resonance imaging studies into the social domain and broaden our view of the spectrum of functions implemented by the dorsal striatum.

Phylogenetic composition of Arctic Ocean archaeal assemblages and comparison with Antarctic assemblages

Archaea assemblages from the Arctic Ocean and Antarctic waters were compared by PCR-denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes amplified using the Archaea-specific primers 344f and 517r. Inspection of the DGGE fingerprints of 33 samples from the Arctic Ocean (from SCICEX submarine cruises in 1995, 1996, and 1997) and 7 Antarctic samples from Gerlache Strait and Dallman Bay revealed that the richness of Archaea assemblages was greater in samples from deep water than in those from the upper water column in both polar oceans. DGGE banding patterns suggested that most of the Archaea ribotypes were common to both the Arctic Ocean and the Antarctic Ocean. However, some of the Euryarchaeota ribotypes were unique to each system. Cluster analysis of DGGE fingerprints revealed no seasonal variation but supported depth-related differences in the composition of the Arctic Ocean Archaea assemblage. The phylogenetic composition of the Archaea assemblage was determined by cloning and then sequencing amplicons obtained from the Archaea-specific primers 21f and 958r. Sequences of 198 clones from nine samples covering three seasons and all depths grouped with marine group I Crenarchaeota (111 clones), marine group II Euryarchaeota (86 clones), and group IV Euryarchaeota (1 clone). A sequence obtained only from a DGGE band was similar to those of the marine group III Euryarchaeota:

A mesoscale iron enrichment in the western subarctic Pacific induces a large centric diatom bloom

We have performed an in situ test of the iron limitation hypothesis in the subarctic North Pacific Ocean. A single enrichment of dissolved iron caused a large increase in phytoplankton standing stock and decreases in macronutrients and dissolved carbon dioxide. The dominant phytoplankton species shifted after the iron addition from pennate diatoms to a centric diatom, Chaetoceros debilis, that showed a very high growth rate, 2.6 doublings per day. We conclude that the bioavailability of iron regulates the magnitude of the phytoplankton biomass and the key phytoplankton species that determine the biogeochemical sensitivity to iron supply of high-nitrate, low-chlorophyll waters.

Respiration in the open ocean

A key question when trying to understand the global carbon cycle is whether the oceans are net sources or sinks of carbon. This will depend on the production of organic matter relative to the decomposition due to biological respiration. Estimates of respiration are available for the top layers, the mesopelagic layer, and the abyssal waters and sediments of various ocean regions. Although the total open ocean respiration is uncertain, it is probably substantially greater than most current estimates of particulate organic matter production. Nevertheless, whether the biota act as a net source or sink of carbon remains an open question.

Phylogenetic composition of bacterioplankton assemblages from the Arctic Ocean

We analyzed the phylogenetic composition of bacterioplankton assemblages in 11 Arctic Ocean samples collected over three seasons (winter-spring 1995, summer 1996, and summer-fall 1997) by sequencing cloned fragments of 16S rRNA genes. The sequencing effort was directed by denaturing gradient gel electrophoresis (DGGE) screening of samples and the clone libraries. Sequences of 88 clones fell into seven major lineages of the domain Bacteria: alpha(36%)-, gamma(32%)-, delta(14%)-, and epsilon(1%)-Proteobacteria; Cytophaga-Flexibacter-Bacteroides spp. (9%); Verrucomicrobium spp. (6%); and green nonsulfur bacteria (2%). A total of 34% of the cloned sequences (excluding clones in the SAR11 and Roseobacter groups) had sequence similarities that were <94% compared to previously reported sequences, indicating the presence of novel sequences. DGGE fingerprints of the selected samples showed that most of the bands were common to all samples in all three seasons. However, additional bands representing sequences related to Cytophaga and Polaribacter species were found in samples collected during the summer and fall. Of the clones in a library generated from one sample collected in spring of 1995, 50% were the same and were most closely affiliated (99% similarity) with Alteromonas macleodii, while 50% of the clones in another sample were most closely affiliated (90 to 96% similarity) with Oceanospirillum sp. The majority of the cloned sequences were most closely related to uncultured, environmental sequences. Prominent among these were members of the SAR11 group. Differences between mixed-layer and halocline samples were apparent in DGGE fingerprints and clone libraries. Sequences related to alpha-Proteobacteria (dominated by SAR11) were abundant (52%) in samples from the mixed layer, while sequences related to gamma-proteobacteria were more abundant (44%) in halocline samples. Two bands corresponding to sequences related to SAR307 (common in deep water) and the high-G+C gram-positive bacteria were characteristic of the halocline samples.

Evidence against dust-mediated control of glacial-interglacial changes in atmospheric CO2

The low concentration of atmospheric CO2 inferred to have been present during glacial periods is thought to have been partly caused by an increased supply of iron-bearing dust to the ocean surface. This is supported by a recent model that attributes half of the CO2 reduction during past glacial stages to iron-stimulated uptake of CO2 by phytoplankton in the Southern Ocean. But atmospheric dust fluxes to the Southern Ocean, even in glacial periods, are thought to be relatively low and therefore it has been proposed that Southern Ocean productivity might be influenced by iron deposited elsewhere-for example, in the Northern Hemisphere-which is then transported south via ocean circulation (similar to the distal supply of iron to the equatorial Pacific Ocean). Here we examine the timing of dust fluxes to the North Atlantic Ocean, in relation to climate records from the Vostok ice core in Antarctica around the time of the penultimate deglaciation (about 130 kyr ago). Two main dust peaks occurred 155 kyr and 130 kyr ago, but neither was associated with the CO2 rise recorded in the Vostok ice core. This mismatch, together with the low dust flux supplied to the Southern Ocean, suggests that dust-mediated iron fertilization of the Southern Ocean did not significantly influence atmospheric CO2 at the termination of the penultimate glaciation.

Glacial/interglacial variations in atmospheric carbon dioxide

Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the ocean's 'biological pump', the sequestration of carbon in the ocean interior by the rain of organic carbon out of the surface ocean, and its effect on the burial of calcium carbonate in marine sediments. Some researchers surmise that the whole-ocean reservoir of algal nutrients was larger during glacial times, strengthening the biological pump at low latitudes, where these nutrients are currently limiting. Others propose that the biological pump was more efficient during glacial times because of more complete utilization of nutrients at high latitudes, where much of the nutrient supply currently goes unused. We present a version of the latter hypothesis that focuses on the open ocean surrounding Antarctica, involving both the biology and physics of that region.

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the 'iron hypothesis'. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.