Oceanic Cd/P ratio and nutrient utilization in the glacial Southern Ocean

Patterns of Element Incorporation in Calcium Carbonate Biominerals Recapitulate Phylogeny for a Diverse Range of Marine Calcifiers

Elemental ratios in biogenic marine calcium carbonates are widely used in geobiology, environmental science, and paleoenvironmental reconstructions. It is generally accepted that the elemental abundance of biogenic marine carbonates reflects a combination of the abundance of that ion in seawater, the physical properties of seawater, the mineralogy of the biomineral, and the pathways and mechanisms of biomineralization. Here we report measurements of a suite of nine elemental ratios (Li/Ca, B/Ca, Na/Ca, Mg/Ca, Zn/Ca, Sr/Ca, Cd/Ca, Ba/Ca, and U/Ca) in 18 species of benthic marine invertebrates spanning a range of biogenic carbonate polymorph mineralogies (low-Mg calcite, high-Mg calcite, aragonite, mixed mineralogy) and of phyla (including Mollusca, Echinodermata, Arthropoda, Annelida, Cnidaria, Chlorophyta, and Rhodophyta) cultured at a single temperature (25°C) and a range of pCO₂ treatments (ca. 409, 606, 903, and 2856 ppm). This dataset was used to explore various controls over elemental partitioning in biogenic marine carbonates, including species-level and biomineralization-pathway-level controls, the influence of internal pH regulation compared to external pH changes, and biocalcification responses to changes in seawater carbonate chemistry. The dataset also enables exploration of broad scale phylogenetic patterns of elemental partitioning across calcifying species, exhibiting high phylogenetic signals estimated from both uni- and multivariate analyses of the elemental ratio data (univariate: λ = 0-0.889; multivariate: λ = 0.895-0.99). Comparing partial R ² values returned from non-phylogenetic and phylogenetic regression analyses echo the importance of and show that phylogeny explains the elemental ratio data 1.4-59 times better than mineralogy in five out of nine of the elements analyzed. Therefore, the strong associations between biomineral elemental chemistry and species relatedness suggests mechanistic controls over element incorporation rooted in the evolution of biomineralization mechanisms.

More efficient North Atlantic carbon pump during the Last Glacial Maximum

During the Last Glacial Maximum (LGM; ~20,000 years ago), the global ocean sequestered a large amount of carbon lost from the atmosphere and terrestrial biosphere. Suppressed CO₂ outgassing from the Southern Ocean is the prevailing explanation for this carbon sequestration. By contrast, the North Atlantic Ocean—a major conduit for atmospheric CO₂ transport to the ocean interior via the overturning circulation—has received much less attention. Here we demonstrate that North Atlantic carbon pump efficiency during the LGM was almost doubled relative to the Holocene. This is based on a novel proxy approach to estimate air–sea CO₂ exchange signals using combined carbonate ion and nutrient reconstructions for multiple sediment cores from the North Atlantic. Our data indicate that in tandem with Southern Ocean processes, enhanced North Atlantic CO₂ absorption contributed to lowering ice-age atmospheric CO₂.

Atmospheric CO₂ is governed by CO₂ gains (e.g., via Southern Ocean outgassing) and losses (e.g., via North Atlantic absorption). Using a novel method to estimate air–sea CO₂ exchange signals, the authors show that North Atlantic CO₂ absorption became more efficient and contributed to lowering atmospheric CO₂ during ice ages.

Episodic release of CO2 from the high-latitude North Atlantic Ocean during the last 135 kyr

Antarctic ice cores document glacial-interglacial and millennial-scale variability in atmospheric pCO₂ over the past 800 kyr. The ocean, as the largest active carbon reservoir on this timescale, is thought to have played a dominant role in these pCO₂ fluctuations, but it remains unclear how and where in the ocean CO₂ was stored during glaciations and released during (de)glacial millennial-scale climate events. The evolution of surface ocean pCO₂ in key locations can therefore provide important clues for understanding the ocean's role in Pleistocene carbon cycling. Here we present a 135-kyr record of shallow subsurface pCO₂ and nutrient levels from the Norwegian Sea, an area of intense CO₂ uptake from the atmosphere today. Our results suggest that the Norwegian Sea probably acted as a CO₂ source towards the end of Heinrich stadials HS1, HS4 and HS11, and may have contributed to the increase in atmospheric pCO₂ at these times.

Glacial-interglacial variations in atmospheric pCO₂ remain unexplained. Here, the authors show that the Norwegian Sea, an modern area of intense CO₂ uptake, acted as a CO₂ source during the terminations of Heinrich stadials 1, 4 and 11, sometimes characterized by rapid increases in atmospheric pCO₂.

Cadmium and phosphate variability during algal blooms of the dinoflagellate Lingulodinium polyedrum in Todos Santos Bay, Baja California, Mexico

Dinoflagellate algal blooms (DABs), with Lingulodinium polyedrum as the dominant species, have increased over the past few years in coastal areas off Baja California, Mexico. Vertical and temporal variability of particulate cadmium (Cdp), dissolved Cd (Cdd), PO4(3-) and Cdd/PO4(3-) were investigated during two intense DABs of L. polyedrum that occurred during the fall of 2011 and 2012 in Todos Santos Bay. Results were then, compared with data gathered in the absence of algal blooms during the autumn of 2013. In both algal blooms, L. polyedrum tended to be concentrated near the surface throughout the duration; however, during DAB 2011 the number of cells was twice as abundant ([10.0 ± 8.0] × 10(5) cells L(-1)) as in DAB 2012 ([5.0 ± 4.4] × 10(5) cells L(-1)). During DAB 2011, Cdp increased significantly (up to 1.02 ± 0.99 nmol kg(-1)) and was positively correlated with the cell abundance of L. polyedrum, suggesting that this dinoflagellate is able to assimilate and concentrate Cdd. Likewise, Cdd (up to 0.71 ± 0.17 nM) increased in the days of highest cell abundance, which could be attributed to uptake and subsequent regeneration of Cdd resulting from the remineralization of organic particulate matter produced during the bloom, as well as with the presence of organic ligands secreted by L. polyedrum that could keep Cdd in solution. During DAB 2011, dissolved Cdd/PO4(3-) ratios exhibited high vertical and temporal variability in the upper 5 m of the water column, but remained virtually constant near the bottom, suggesting a depth-dependent decoupling between these two dissolved components during the bloom development. Given the observed differences in the vertical and temporal variability of Cdd, Cdp, and PO4(3-) between these two intense DABs, we propose the existence of an abundance threshold of approximately 10(6) cells L(-1) of L. polyedrum above which Cd and PO4(3-) significantly increased due to remineralization in coastal waters during the bloom development.

Goldilocks and the three inorganic equilibria: how Earth's chemistry and life coevolve to be nearly in tune

Life and the chemical environment are united in an inescapable feedback cycle. The periodic table of the elements essential for life has transformed over Earth's history, but, as today, evolved in tune with the elements available in abundance in the environment. The most revolutionary time in life's history was the advent and proliferation of oxygenic photosynthesis which forced the environment towards a greater degree of oxidation. Consideration of three inorganic chemical equilibria throughout this gradual oxygenation prescribes a phased release of trace metals to the environment, which appear to have coevolved with employment of these new chemicals by life. Evolution towards complexity was chemically constrained, and changes in availability of notably Fe, Zn and Cu paced the systematic development of complex organisms. Evolving life repeatedly catalysed its own chemical challenges via the unwitting release of new and initially toxic chemicals. Ultimately, the harnessing of these allowed life to advance to greater complexity, though the mechanism responsible for translating novel chemistry to heritable use remains elusive. Whether a chemical acts as a poison or a nutrient lies both in the dose and in its environmental history.

Undocumented water column sink for cadmium in open ocean oxygen-deficient zones

Cadmium (Cd) is a micronutrient and a tracer of biological productivity and circulation in the ocean. The correlation between dissolved Cd and the major algal nutrients in seawater has led to the use of Cd preserved in microfossils to constrain past ocean nutrient distributions. However, linking Cd to marine biological processes requires constraints on marine sources and sinks of Cd. Here, we show a decoupling between Cd and major nutrients within oxygen-deficient zones (ODZs) in both the Northeast Pacific and North Atlantic Oceans, which we attribute to Cd sulfide (CdS) precipitation in euxinic microenvironments around sinking biological particles. We find that dissolved Cd correlates well with dissolved phosphate in oxygenated waters, but is depleted compared with phosphate in ODZs. Additionally, suspended particles from the North Atlantic show high Cd content and light Cd stable isotope ratios within the ODZ, indicative of CdS precipitation. Globally, we calculate that CdS precipitation in ODZs is an important, and to our knowledge a previously undocumented marine sink of Cd. Our results suggest that water column oxygen depletion has a substantial impact on Cd biogeochemical cycling, impacting the global relationship between Cd and major nutrients and suggesting that Cd may be a previously unidentified tracer for water column oxygen deficiency on geological timescales. Similar depletions of copper and zinc in the Northeast Pacific indicate that sulfide precipitation in ODZs may also have an influence on the global distribution of other trace metals.

Proteomic responses of oceanic Synechococcus WH8102 to phosphate and zinc scarcity and cadmium additions

Synechococcus sp. WH 8102 is a motile marine cyanobacterium isolated originally from the Sargasso Sea. To test the response of this organism to cadmium (Cd), generally considered a toxin, cultures were grown in a matrix of high and low zinc (Zn) and phosphate (PO₄³⁻) and were then exposed to an addition of 4.4 pM free Cd²⁺ at mid-log phase and harvested after 24 h. Whereas Zn and PO₄³⁻ had little effect on overall growth rates, in the final 24 h of the experiment three growth effects were noticed: (i) low PO₄³⁻ treatments showed increased growth rates relative to high PO₄³⁻ treatments, (ii) the Zn/high PO₄³⁻ treatment appeared to enter stationary phase, and (iii) Cd increased growth rates further in both the low PO₄³⁻ and Zn treatments. Global proteomic analysis revealed that: (i) Zn appeared to be critical to the PO₄³⁻ response in this organism, (ii) bacterial metallothionein (SmtA) appears correlated with PO₄³⁻ stress-associated proteins, (iii) Cd has the greatest influence on the proteome at low PO₄³⁻ and Zn, (iv) Zn buffered the effects of Cd, and (v) in the presence of both replete PO₄³⁻ and added Cd the proteome showed little response to the presence of Zn. Similar trends in alkaline phosphate (ALP) and SmtA suggest the possibility of a Zn supply system to provide Zn to ALP that involves SmtA. In addition, proteome results were consistent with a previous transcriptome study of PO₄³⁻ stress (with replete Zn) in this organism, including the greater relative abundance of ALP (PhoA), ABC phosphate binding protein (PstS) and other proteins. Yet with no Zn in this proteome experiment the PO₄³⁻ response was quite different including the greater relative abundance of five hypothetical proteins with no increase in PhoA or PstS, suggesting that Zn nutritional levels are connected to the PO₄³⁻ response in this cyanobacterium. Alternate ALP PhoX (Ca) was found to be a low abundance protein, suggesting that PhoA (Zn, Mg) may be more environmentally relevant than PhoX.

Baseline benthic foraminiferal assemblages and habitat conditions in a sub-Arctic region of increasing petroleum development

The aim of this study is to establish pre-impact baseline conditions for an Arctic region where petroleum activities are projected to increase in the coming decades. We characterize the spatial distribution of living benthic foraminifera in the Tromsøflaket-Ingøydjupet region of the Barents Sea and relate this to sediment properties and their associated metal concentrations. Metal concentrations of the sediments did not exceed threshold levels of harmful environmental effects, indicating that the area exhibits pre-impact baseline conditions. Foraminiferal assemblages reflect the pristine environment. Epifaunal species dominate in Tromsøflaket, a high energy environment characterized by coarse grained sediments. Infaunal species dominate in Ingøydjupet, a low energy environment characterized by fine grained sediments. Metal concentrations were slightly elevated in the fine grained sediments from Ingøydjupet which suggest that these areas may in the future serve as trapping zones for contaminants associated with discharges from nearby petroleum sites.

Continuous flow analysis method for determination of dissolved reactive phosphorus in ice cores

Phosphorus (P) is an essential macronutrient for all living organisms. Phosphorus is often present in nature as the soluble phosphate ion PO4(3-) and has biological, terrestrial, and marine emission sources. Thus PO4(3-) detected in ice cores has the potential to be an important tracer for biological activity in the past. In this study a continuous and highly sensitive absorption method for detection of dissolved reactive phosphorus (DRP) in ice cores has been developed using a molybdate reagent and a 2-m liquid waveguide capillary cell (LWCC). DRP is the soluble form of the nutrient phosphorus, which reacts with molybdate. The method was optimized to meet the low concentrations of DRP in Greenland ice, with a depth resolution of approximately 2 cm and an analytical uncertainty of 1.1 nM (0.1 ppb) PO4(3-). The method has been applied to segments of a shallow firn core from Northeast Greenland, indicating a mean concentration level of 2.74 nM (0.26 ppb) PO4(3-) for the period 1930-2005 with a standard deviation of 1.37 nM (0.13 ppb) PO4(3-) and values reaching as high as 10.52 nM (1 ppb) PO4(3-). Similar levels were detected for the period 1771-1823. Based on impurity abundances, dust and biogenic particles were found to be the most likely sources of DRP deposited in Northeast Greenland.

The "white ocean" hypothesis: a late pleistocene southern ocean governed by coccolithophores and driven by phosphorus

Paleoproductivity is a critical component in past ocean biogeochemistry, but accurate reconstructions of productivity are often hindered by limited integration of proxies. Here, we integrate geochemical (phosphorus) and micropaleontological proxies at millennial timescales, revealing that the coccolithophore record in the Subantarctic zone of the South Atlantic Ocean is driven largely by variations in marine phosphorus availability. A quantitative micropaleontological and geochemical analysis carried out in sediments retrieved from Ocean Drilling Program Site 1089 (Subantarctic Zone) reveals that most of the export productivity in this region over the last 0.5 my was due to coccolithophores. Glacial periods were generally intervals of high productivity, with productivity reaching a peak at terminations. Particularly high productivity was observed at Termination V and Termination IV, events that are characterized by high abundance of coccolithophores and maxima in the phosphorus/titanium and strontium/titanium records. We link the increase in productivity both to regional oceanographic phenomena, i.e., the northward displacement of the upwelling cell of the Antarctic divergence when the ice-sheet expanded, and to the increase in the inventory of phosphorus in the ocean due to enhanced transfer of this nutrient from continental margins during glacial lowstands in sea level. The Mid-Brunhes interval stands out from the rest of the record, being dominated by the small and highly calcified species Gephyrocapsa caribbeanica that provides most of the carbonate in these sediments. This likely represents higher availability of phosphorus in the surface ocean, especially in mesotrophic and oligotrophic zones. Under these condition, some coccolithophore species developed an r-strategy (opportunistic species; growth rate maximized) resulting in the bloom of G. caribbeanica. These seasonal blooms of may have induced "white tides" similar to those observed today in Emiliania huxleyi.

Disassembling iron availability to phytoplankton

The bioavailability of iron to microorganisms and its underlying mechanisms have far reaching repercussions to many natural systems and diverse fields of research, including ocean biogeochemistry, carbon cycling and climate, harmful algal blooms, soil and plant research, bioremediation, pathogenesis, and medicine. Within the framework of ocean sciences, short supply and restricted bioavailability of Fe to phytoplankton is thought to limit primary production and curtail atmospheric CO₂ drawdown in vast ocean regions. Yet a clear-cut definition of bioavailability remains elusive, with elements of iron speciation and kinetics, phytoplankton physiology, light, temperature, and microbial interactions, to name a few, all intricately intertwined into this concept. Here, in a synthesis of published and new data, we attempt to disassemble the complex concept of iron bioavailability to phytoplankton by individually exploring some of its facets. We distinguish between the fundamentals of bioavailability – the acquisition of Fe-substrate by phytoplankton – and added levels of complexity involving interactions among organisms, iron, and ecosystem processes. We first examine how phytoplankton acquire free and organically bound iron, drawing attention to the pervasiveness of the reductive uptake pathway in both prokaryotic and eukaryotic autotrophs. Turning to acquisition rates, we propose to view the availability of various Fe-substrates to phytoplankton as a spectrum rather than an absolute “all or nothing.” We then demonstrate the use of uptake rate constants to make comparisons across different studies, organisms, Fe-compounds, and environments, and for gaging the contribution of various Fe-substrates to phytoplankton growth in situ. Last, we describe the influence of aquatic microorganisms on iron chemistry and fate by way of organic complexation and bio-mediated redox transformations and examine the bioavailability of these bio-modified Fe species.

Ocean nutrient ratios governed by plankton biogeography

The major nutrients nitrate and phosphate have one of the strongest correlations in the sea, with a slope similar to the average nitrogen (N) to phosphorus (P) content of plankton biomass (N/P = 16:1). The processes through which this global relationship emerges despite the wide range of N/P ratios at the organism level are not known. Here we use an ocean circulation model and observed nutrient distributions to show that the N/P ratio of biological nutrient removal varies across latitude in Southern Ocean surface waters, from 12:1 in the polar ocean to 20:1 in the sub-Antarctic zone. These variations are governed by regional differences in the species composition of the plankton community. The covariation of dissolved nitrate and phosphate is maintained by ocean circulation, which mixes the shallow subsurface nutrients between distinct biogeographic provinces. Climate-driven shifts in these marine biomes may alter the mean N/P ratio and the associated carbon export by Southern Ocean ecosystems.

Role of marine biology in glacial-interglacial CO2 cycles

It has been hypothesized that changes in the marine biological pump caused a major portion of the glacial reduction of atmospheric carbon dioxide by 80 to 100 parts per million through increased iron fertilization of marine plankton, increased ocean nutrient content or utilization, or shifts in dominant plankton types. We analyze sedimentary records of marine productivity at the peak and the middle of the last glacial cycle and show that neither changes in nutrient utilization in the Southern Ocean nor shifts in plankton dominance explain the CO2 drawdown. Iron fertilization and associated mechanisms can be responsible for no more than half the observed drawdown.

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.