Evidence for elevated alkalinity in the glacial Southern Ocean

An increase in whole ocean alkalinity during glacial periods could account, in part, for the drawdown of atmospheric CO2 into the ocean. Such an increase was inevitable due to the near elimination of shelf area for the burial of coral reef alkalinity. We present evidence, based on downcore measurements of benthic foraminiferal B/Ca and Mg/Ca from a core in the Weddell Sea, that the deep ocean carbonate ion concentration, [CO32-], was elevated by similar to 25 mu mol/kg during each glacial period of the last 800 kyr. The heterogeneity of the preservation histories in the different ocean basins reflects control of the carbonate chemistry of the deep glacial ocean in the Atlantic and Pacific by the changing ventilation and chemistry of Weddell Sea waters. These waters are more corrosive than interglacial northern sourced waters but not as undersaturated as interglacial southern sourced waters. Our inferred increase in whole ocean alkalinity can be reconciled with reconstructions of glacial saturation horizon depth and the carbonate budget if carbonate burial rates also increased above the saturation horizon as a result of enhanced pelagic calcification. The Weddell records display low [CO32-] during deglaciations and peak interglacial warmth, coincident with maxima in percent CaCO3 in the Atlantic and Pacific oceans. Should the burial rate of alkalinity in the more alkaline glacial deep waters outstrip the rate of alkalinity supply, then pelagic carbonate production by the coccolithophores at the end of the glacial maximum could drive a decrease in ocean [CO32-] and act to trigger the deglacial rise in pCO(2)

The role of Rubisco kinetics and pyrenoid morphology in shaping the CCM of haptophyte microalgae

The haptophyte algae are a cosmopolitan group of primary producers that contribute significantly to the marine carbon cycle and play a major role in paleo-climate studies. Despite their global importance, little is known about carbon assimilation in haptophytes, in particular the kinetics of their Form 1D CO2-fixing enzyme, Rubisco. Here we examine Rubisco properties of three haptophytes with a range of pyrenoid morphologies (Pleurochrysis carterae, Tisochrysis lutea, and Pavlova lutheri) and the diatom Phaeodactylum tricornutum that exhibit contrasting sensitivities to the trade-offs between substrate affinity (Km) and turnover rate (kcat) for both CO2 and O2. The pyrenoid-containing T. lutea and P. carterae showed lower Rubisco content and carboxylation properties (KC and kC cat) comparable with those of Form 1D-containing non-green algae. In contrast, the pyrenoid-lacking P. lutheri produced Rubisco in 3-fold higher amounts, and displayed a Form 1B Rubisco kC cat–KC relationship and increased CO2/O2 specificity that, when modeled in the context of a C3 leaf, supported equivalent rates of photosynthesis to higher plant Rubisco. Correlation between the differing Rubisco properties and the occurrence and localization of pyrenoids with differing intracellular CO2:O2 microenvironments has probably influenced the divergent evolution of Form 1B and 1D Rubisco kineticsAMCH was funded through a Clarendon Scholarship, Oxford and ANU visiting scholar (CE140100015). Funding for JNY and SMW was provided through Australian Research Council Grant CE14010001. RES was funded through the ARC DECRA scheme (DE13010760) and REMR was funded through an ERC Starting Grant (SP2-GA-2008-200915)

Controls on stable isotope and trace metal uptake in Neogloboquadrina pachyderma (sinistral) from an Antarctic sea-ice environment

The polar foraminifera Neogloboquadrina pachyderma (sinistral) dominates assemblages from the high latitude Southern Ocean, which plays a key role in determining past climate due to the tight linkage between Antarctic temperature and atmospheric CO2. Here, we use N. pachyderma (s.) harvested from sediment traps off the West Antarctic Peninsula to construct a seasonal time series for the calibration of calcite proxies in a high latitude seasonal sea-ice environment where temperature is decoupled from other environmental parameters. We have used a combination of δ18OCaCO3 and δ 13CCaCO3 to decipher the calcification temperature and salinity, which reflect that N. pachyderma (s.) live in surface waters throughout the year, and at the ice–water interface in austral winter. Further, our results demonstrate that the uptake of trace metals into N. pachyderma (s.) calcite is influenced by secondary environmental conditions in addition to temperature during periods of sea-ice cover. We propose an elevated carbonate ion concentration at the ice– water interface resulting from biological utilisation of CO2 could influence calcification in foraminifera. Our calculations suggest that for N. pachyderma (s.) Mg/Ca, Sr/Ca ratios and Li/Ca ratios are linear functions of calcification temperature and [CO3 2−]. N. pachyderma (s.) Mg/Ca ratios exhibit temperature sensitivity similar to previous studies (~10–20%/°C) and a sensitivity to [CO32−] of ~1%/μmol kg−1. Sr/Ca ratios are less sensitive to environmental parameters, exhibiting ~5% increase/°C and ~0.5%/10 μmol kg−1. The relationship between Li/Ca ratios and both temperature and [CO32−] is less significant with ~10% increase in Li/Ca ratio/°C and 10 μmol kg−1.We show how a multi-proxy approach could be used to constrain past high latitude surface water temperature and [CO3 2−]

Numerical issues in mold filling simulations of liquid composites processing

Rubisco, the most abundant enzyme on the Earth and responsible for all photosynthetic carbon fixation, is often thought of as a highly conserved and sluggish enzyme. Yet, different algal Rubiscos demonstrate a range of kinetic properties hinting at a history of evolution and adaptation. Here, we show that algal Rubisco has indeed evolved adaptively during ancient and distinct geological periods. Using DNA sequences of extant marine algae of the red and Chromista lineage, we define positive selection within the large subunit of Rubisco, encoded by rbcL, to occur basal to the radiation of modern marine groups. This signal of positive selection appears to be responding to changing intracellular concentrations of carbon dioxide (CO2) triggered by physiological adaptations to declining atmospheric CO2. Within the ecologically important Haptophyta (including coccolithophores) and Bacillariophyta (diatoms), positive selection occurred consistently during periods of falling Phanerozoic CO2 and suggests emergence of carbon-concentrating mechanisms. During the Proterozoic, a strong signal of positive selection after secondary endosymbiosis occurs at the origin of the Chromista lineage (approx. 1.1 Ga), with further positive selection events until 0.41 Ga, implying a significant and continuous decrease in atmospheric CO2 encompassing the Cryogenian Snowball Earth events. We surmise that positive selection in Rubisco has been caused by declines in atmospheric CO2 and hence acts as a proxy for ancient atmospheric CO2

On the potential role of marine calcifiers in glacial-interglacial dynamics

[1] Ice core measurements have revealed a highly asymmetric cycle in Antarctic temperature and atmospheric CO2 over the last 800 kyr. Both CO2 and temperature decrease over 100 kyr going into a glacial period and then rise steeply over less than 10 kyr at the end of a glacial period. There does not yet exist wide agreement about the causes of this cycle or about the origin of its shape. Here we explore the possibility that an ecologically driven oscillator plays a role in the dynamics. A conceptual model describing the interaction between calcifying plankton and ocean alkalinity shows interesting features: (i) It generates an oscillation in atmospheric CO2 with the characteristic asymmetric shape observed in the ice core record, (ii) the system can transform a sinusoidal Milankovitch forcing into a sawtooth-shaped output, and (iii) there are spikes of enhanced calcifier productivity at the glacial-interglacial transitions, consistent with several sedimentary records. This suggests that ecological processes might play an active role in the observed glacial-interglacial cycles

The effect of ocean alkalinity and carbon transfer on deep-sea carbonate ion concentration during the past five glacial cycles

Glacial–interglacial deep Indo-Pacific carbonate ion concentration ([CO32−]) changes were mainly driven by two mechanisms that operated on different timescales: 1) a long-term increase during glaciation caused by a carbonate deposition reduction on shelves (i.e., the coral reef hypothesis), and 2) transient carbonate compensation responses to deep ocean carbon storage changes. To investigate these mechanisms, we have used benthic foraminiferal B/Ca to reconstruct deep-water [CO32−] in cores from the deep Indian and Equatorial Pacific Oceans during the past five glacial cycles. Based on our reconstructions, we suggest that the shelf-to-basin shift of carbonate deposition raised deep-water [CO32−], on average, by 7.3 ± 0.5 (SE) μmol/kg during glaciations. Oceanic carbon reorganisations during major climatic transitions caused deep-water [CO32−] deviations away from the long-term trend, and carbonate compensation processes subsequently acted to restore the ocean carbonate system to new steady state conditions. Deep-water [CO32−] showed similar patterns to sediment carbonate content (%CaCO3) records on glacial–interglacial timescales, suggesting that past seafloor %CaCO3 variations were dominated by deep-water carbonate preservation changes at our studied site

A top-down and bottom-up comparison of paleoproductivity proxies: calcareous nannofossil Sr/Ca ratios and benthic foraminiferal accumulation rates

We investigate whether a relationship exists between organic matter production at the sea surface, recorded by nannofossil carbonate Sr/Ca, and its consumption on the seafloor, measured by benthic foraminiferal accumulation rates (BFAR). A mid-Pliocene (3.67-3.90 Ma) section of Ocean Drilling Program Site 926 in the northwestern tropical Atlantic was sampled, where previous work established that the calcareous nannoplankton assemblages vary with insolation linked changes in surface water productivity. Our results reveal that coarse fraction nannofossil Sr/Ca varies with changes in assemblage composition and may be predominantly controlled by the geochemistry of the warm water oligotrophic genus Discoaster. BFAR also have a positive relationship with Sr/Ca and Discoaster abundances, implying times of relatively low surface water nutrients coincide with enhanced BFAR. This result is opposite of what one would expect given the assumption of a direct relationship between primary and export production. We speculate that the BFAR are stimulated by enhanced organic carbon export associated with ballasting by nannofossil assemblages dominated volumetrically by large, robust taxa such as Discoaster species. These results highlight the complexity of interpretations of bulk nannofossil Sr/Ca ratios, as well as BFAR data, with respect to paleoproductivity

A synthesis of marine sediment core delta C data over the last 150 000 years

The isotopic composition of carbon, ?13C, in seawater is used in reconstructions of ocean circulation, marine productivity, air-sea gas exchange, and biosphere carbon storage. Here, a synthesis of ?13C measurements taken from foraminifera in marine sediment cores over the last 150 000 years is presented. The dataset comprises previously published and unpublished data from benthic and planktonic records throughout the global ocean. Data are placed on a common ?18O age scale suitable for examining orbital timescale variability but not millennial events, which are removed by a 10 ka filter. Error estimates account for the resolution and scatter of the original data, and uncertainty in the relationship between ?13C of calcite and of dissolved inorganic carbon (DIC) in seawater. This will assist comparison with ?13C of DIC output from models, which can be further improved using model outputs such as temperature, DIC concentration, and alkalinity to improve estimates of fractionation during calcite formation