ALBERT

Description: Accurately quantifying deep-sea calcite dissolution is crucial for understanding the role of the marine carbonate system in regulating atmospheric pCO2 over millennia. We compare a foraminifer-fragmentation-based calcite dissolution proxy (Globorotalia menardii fragmentation index (MFI)) to Mg/Ca, Sr/Ca, and Mg/Sr in several species of deep dwelling planktonic foraminifers. We conducted microfossil and geochemical analyses on the same core top samples taken at different depths on the Ontong Java Plateau to maximize the dissolution signal and minimize the temperature overprint on our data. We also compare elemental ratios from planktonic foraminifer tests to modern bottom water [CO3]2- undersaturation and model-derived estimates of percent calcite dissolved in deep-sea sediments. We find clear linear decreases in Mg/Ca or Mg/Sr in G. menardii and Pulleniatina obliquiloculata with increasing (1) bottom water [CO3]2- undersaturation, (2) percent calcite dissolved in sediments calculated with biogeochemical modeling, (3) MFI, and (4) percent calcite dissolved derived from MFI. These findings lend further support to MFI as a calcite dissolution proxy for deep-sea sediments. In contrast, we find no significant correlation between Sr/Ca and independent dissolution indicators. Our results suggest that Mg/Ca and Mg/Sr from deep dwelling foraminifers could potentially be used as calcite dissolution proxies in combination with independent water temperature estimates. Likewise, establishing the relationship between MFI and dissolution-induced changes in the Mg/Ca of surface-dwelling foraminifers could provide a tool to correct Mg/Ca–derived sea surface temperature reconstructions for calcite dissolution.

Description: Mg/Ca in planktonic foraminifers carries two main signals: calcification temperature and postdepositional test dissolution. Shell dissolution thus distorts water temperature reconstructions made with Mg/Ca in foraminifers. This problem could be resolved by quantifying the impact of carbonate dissolution on Mg/Ca with an independent, temperature-insensitive deep-sea calcite dissolution proxy, such as the Globorotalia menardii fragmentation index (MFI). To test the validity of this approach, we measured Mg/Ca in the tests of several planktonic foraminifers and MFI in core tops collected over a wide geographic region of the tropical Pacific and covering a wide range of deep-sea calcite dissolution and seawater temperature. We confirm that Mg/Ca from different species have different susceptibility to temperature and dissolution. Mg/Ca in surface-dwelling Globigerina bulloides is controlled by calcification temperature and is largely unaffected by carbonate dissolution estimated from MFI. In contrast, Mg/Ca in deeper dwelling G. menardii is minimally sensitive to temperature and dominantly affected by dissolution. Mg/Ca in Neogloboquadrina dutertrei and Pulleniatina obliquiloculata are significantly affected by both temperature and dissolution, and MFI can be effectively used to correct temperature estimates from these species for calcite dissolution. Additional variables besides temperature and dissolution appear to control Mg/Ca in Globorotalia tumida, and their identification is a prerequisite for interpreting elemental shell composition in this species. Combining down-core measurements of Mg/Ca in multiple foraminifer species with MFI provides a powerful tool for reconstructing past changes in the upper water column temperature structure in the tropical Pacific.

Description: Finding the ideal deep-sea CaCO3 dissolution proxy is essential for quantifying the role of the marine carbonate system in regulating atmospheric pCO2 over millennia. We explore the potential of using the Globorotalia menardii fragmentation index (MFI) and size-normalized foraminifer shell weight (SNSW) as complementary indicators of deep-sea CaCO3 dissolution. MFI has strong correlations with bottom water [CO3]2-, modeled estimates of percent CaCO3 dissolved, and Mg/Ca in Pulleniatina obliquiloculata in core top samples along a depth transect on the Ontong Java Plateau (OJP) where surface ocean temperature variation is minimal. SNSW of P. obliquiloculata and Neogloboquadrina dutertrei have weak correlations with MFI-based percent dissolved, Mg/Ca in P. obliquiloculata shells and bottom water [CO3]2- on the OJP. In core top samples from the eastern equatorial Pacific (EEP), SNSW of P. obliquiloculata has moderate to strong correlations with both MFI-based percent CaCO3 dissolved estimates and surface ocean environmental parameters. SNSW of N. dutertrei shells shows a latitudinal distribution in the EEP and a moderately strong correlation with MFI-based percent dissolved estimates when samples from the equatorial part of the region are excluded. Our results suggest that there may potentially be multiple genotypes of N. dutertrei in the EEP which may be reflected in their shell weight. MFI-based percent CaCO3 dissolved estimates have no quantifiable relationship with any surface ocean environmental parameter in the EEP. Thus MFI acts as a reliable quantitative CaCO3 dissolution proxy insensitive to environmental biases within calcification waters of foraminifers.

Description: Previous paleoceanographic applications of the N isotopes in the eastern equatorial Pacific have used the N isotopic composition of the bulk sediment, which can be biased by diagenetic alteration or foreign N input. To avoid these biases, we measured foraminifera shell-bound d15N (FB-d15N) on the two species Neogloboquadrina dutertrei and Neogloboquadrina incompta in two sediment cores extending back to the last ice age. The datafile contains FB-d15N data measured on the two sediment cores ME0005-24JC (0°1.3' N, 86°27.8' W, 2941m) and ME0005-27JC (1°51.2' S, 82°47.2'W, 2203m) from the eastern equatorial Pacific, as well as updated age models for the two sediment cores. Moreover, it contains estimated changes in Pacific oxygen concentration from the LGM to the Holocene. The age models for both sediment cores have been updated by Dubois et al. (2014) and are based on (1) radiocarbon ages measured on the planktonic foraminifera N. dutertrei by accelerator mass spectrometry, (2) correlation of benthic foraminifera oxygen isotopes to the LR04 stack and (3) the identification of the Los Chocoyos Ash Layer in the sediment cores. In core ME0005-27JC, three additional 14C dates on N. dutertrei from Mekik (2014) were included. All radiocarbon ages were calibrated with Calib 7.1. and the marine calibration curve MARINE13, assuming a reservoir age of 467 years as given in Dubois et al. (2014). Ages were linearly interpolated between the stratigraphic tie points. Foraminifera-bound d15N (FB-d15N) was measured with the “persulfate-denitrifier” technique (Ren et al., 2009; Straub et al., 2013). In short, ~3-5 mg of foraminifera (N. dutertrei and N. incompta from the 300-600µm size fraction) were picked, cut open with a scalpel and underwent a chemical cleaning. The organic N bound within the calcite was then released by dissolution with HCl and converted to nitrate in a basic potassium persulfate solution. The nitrate concentration of the solution was determined by chemiluminescence, and an aliquot of the nitrate solution equivalent to 5nmol of N was converted to nitrous oxide (N2O) by denitrifying bacteria. The N isotopic composition of the N2O was measured with a custom continuous-flow system for N2O extraction and purification on-line to a Thermo MAT253 stable isotope mass spectrometer and referenced to air N2 using the international nitrate standards IAEA-N3 and USGS-34. The FB-d15N data were then corrected for the contribution of the oxidation procedural blank with an in-house aminocaproic acid standard of known isotopic composition. Changes in Pacific oxygen concentration from the LGM to the Holocene were calculated based on solubility changes as well as CYCLOPS box model results of Hain et al., (2010). Changes in oxygen saturation result from changes in temperature and salinity; changes in oxygen utilization result from a glacial shoaling of the Atlantic Meridional Overturning Circulation, enhanced nutrient consumption due to Subantarctic iron fertilization, reduced Antarctic surface-to-deep exchange and more complete Antarctic nutrient consumption. Oxygen utilization is calculated using O2:Pregenerated of -170:1 (Anderson and Sarmiento, 1994).