R. E. M. Rickaby

Evidence from the high-latitude North Atlantic for variations in Antarctic Intermediate water flow during the last deglaciation.

We present evidence that the characteristic chemical signature (based on coupled benthic foraminiferal Cd/Ca and d13C) of Antarctic Intermediate waters (AAIW) penetrated throughout the intermediate depths of the Atlantic basin to the high-latitude North Atlantic during the abrupt cooling events of the last deglaciation: Heinrich 1 and the Younger Dryas. AAIW may play the dynamic counterpart to the ‘‘bipolar seesaw’’ when near-freezing salty bottom waters from the Antarctic (AABW) sluggishly ventilate the deep ocean. Our data reinforce the concept that interglacial circulation is stabilized by salinity feedbacks between salty northern sourced deep waters (NADW) and fresh southern sourced waters (AABW and AAIW). Further, the glacial ocean may be susceptible to the more finely balanced relative densities of NADW and AAIW, due to either freshwater input or a reversal of the salinity gradient, such that the ocean is poised for NADW collapse via a negative salinity feedback. The unstable climate of the glacial period and its termination may arise from the closer competition for ubiquity at intermediate depths between northern and southern sourced intermediate waters.

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).

A method for precise analysis of trace element/calcium ratios in carbonate samples using quadrupole inductively coupled plasma mass spectrometry

An improved method has been developed using a quadrupole inductively coupled plasma mass spectrometer (Q-ICP-MS) to analyze carbonate samples for multiple trace element to calcium ratios. Problems alleviating shorter-term drift have been overcome by manipulating the instrument to bracket each trace metal measurement with a calcium measurement, while longer-term drift is corrected by bracketing each sample analysis with external matrix-matched standards, of typical foram composition. Our method is optimized for measuring the ratios, Sr/Ca, Mg/Ca, Zn/Ca, Cd/Ca, Ba/Ca, Mn/Ca, and U/Ca, from a single carbonate sample but could be adapted to include a range of alternative ratios. Evaluation has shown the method to be extremely precise. At less than 1 RSD (1σ) for 3 trace metal to calcium ratios, we have achieved comparable precision to a sector field ICP-MS instrument and an advance over previous methods using the Q-ICP-MS instrument. Precision (RSD) for each ratio, based on 96 sample analyses in 5 batches made over a period of 9 months using a 100 mg/L Ca standard, is 2.2% for U/Ca, 1.7% for Cd/Ca, 1.9% for Zn/Ca, 1.1% for Ba/Ca, 0.7% for Mn/Ca, and 0.7% and 0.6% for Sr/Ca and Mg/Ca, respectively. Accuracy of the method is confirmed by comparing standard ratios with those obtained from an ICP-AES instrument and by comparing sample analyses of NEAP4B material from the North Atlantic with data measured using thermal ionization mass spectrometry (TIMS) and flame atomic absorption spectroscopy (FAAS).

Expanded oxygen minimum zones during the late Paleocene‐early Eocene: Hints from multiproxy comparison and ocean modeling

Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between deoxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore water redox conditions in the South Atlantic and Southern Indian Oceans and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5–2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low-oxygen sites, than at well-oxygenated modern sites, indicating higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (Community Climate System Model Version 3), but the simulated absolute dissolved O2 values are low (< ~35 µmol/kg), while higher O2 values (~60–100 µmol/kg) were obtained in an Earth system model (Grid ENabled Integrated Earth system model). Multiproxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved.

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.