NOAA/WDS Paleoclimatology - Eastern Equatorial Pacific 100KYr Alkenone SST Reconstructions

STUDY NOTES: The data set contains alkenone-based SST estimates, bulk sediment d15N and Corg content from 4 cores from the eastern equatorial Pacific covering the late Quaternary, in addition to the opal record and the residual opal record (with variability >10 kyr removed) from ME0005A-24JC. The data from core ME0005A-24JC are presented against 2 age models: scenario 1 and scenario 2, as discussed in the original reference. Cores ME0005A-24JC, ME0005A-27JC and TR163-31P were analyzed for alkenone unsaturation at Dalhousie University following standard laboratory procedures detailed by Kienast et al. (2006). Sea-surface temperature estimates (UK'37 SST) were calculated from the ratio of the concentration of the di and triunsaturated alkenones, using the calibration of Prahl et al. (1988). The upper 500 cm of ME0005A-24JC were analyzed at WHOI following the same procedure and were previously published by Kienast et al. (2006). These data were shifted by +0.027 UK'37 units because of a laboratory offset determined from 10 replicate samples. Cores TR163-19P was analyzed at Brown University following similar laboratory procedures detailed in Herbert et al. (1998). Results from the last 25 kyr BP of cores ME0005A-27JC, TR163-19P and TR163-31P were previously published in Dubois et al. (2009). The sedimentary d15N composition of all cores was analyzed at UBC, Vancouver, following standard procedures. Total carbon was determined with an elemental analyzer. Inorganic (i.e., carbonate mineral) carbon was determined in a carbon dioxide coulometer. Organic carbon was estimated by subtracting inorganic from total carbon. Results for the last 35 kyr have been published by Kienast et al. (2006, 2007). Biogenic opal of core ME0005A-24JC (>510 cm) was determined colorimetrically following alkaline extraction of silica (Mortlock and Froelich, 1989). The upper 510 cm were analyzed at UBC following the same procedure and were previously published by Kienast et al. (2007). Age models for Marine Isotopic Stage 1 and 2 (MIS1 and MIS2, i.e. 0-25 kyr) in cores ME0005A-27JC, TR163-19P and TR163-31P were adopted as previously published (see Kienast et al., 2007). We present here an updated MIS1-2 age model of core ME0005A-24JC based on 4 radiocarbon dates on Neogloboquadrina dutertrei published by Kienast et al. (2007) and 2 additional dates on N. dutertrei published by Kusch et al. (2010). Radiocarbon ages were calibrated to calendar years using the software CALIB 6.0 (Stuiver and Reimer, 1993; deltaR = 167 ± 106 yr), and the MARINE09 calibration data set (Reimer et al., 2009). Note that the use of this new calibration data set (MARINE09) leads to small deviations (<600 yr) from the calendar ages published by Kienast et al. (2007) using the MARINE04 (Hughen et al., 2004) calibration data set. Because of the difficulty in dating sediments from the equatorial Pacific older than 50,000 yr, we created two different age models based on the hypotheses under investigation. Cores TR163-31P, TR163-19P and ME0005A-27JC were graphically correlated to ME0005A-24JC for the MIS3-4 interval using the software AnalySeries (Paillard et al., 1996). We based our correlation on a number of clear features in the d15N and opal records, making sure not to violate the MIS4 benthic d18O transitions. Finally, based on multiple geochemical proxies (not shown), we identify an ash layer at a depth of 9.8 m ME0005A-24JC, 4.31 m ME0005A-27JC, 3.10 m in TR163-19P and 6.61 m in TR163-31P, which we assume to be the Los Chocoyos Ash Layer. This Ash Layer was previously observed in the Gulf of Mexico and equatorial Pacific and has been dated to 84,000 yr BP on the basis of oxygen isotope stratigraphy, biostratigraphy and Pa-Th data (Drexler et al., 1980).
ABSTRACT SUPPLIED BY ORIGINATOR: In this study, we use records of nitrogen isotope ratios (d15N), UK'37 temperature estimates, organic carbon and opal percentages from high-resolution sediment cores located in the eastern equatorial Pacific (EEP) to explore the mechanisms linking millennial-scale changes in low-latitude sea surface temperature, water column denitrification and surface productivity to the timing of northern or southern polar climate during the last 100,000 yr. Our results support a hypothesis that the Southern Hemisphere, and its connection to the low latitudes via shallow subsurface ocean circulation, has a primary influence on the biogeochemistry of the EEP. In addition, our results suggest that, during the last glacial stage, denitrification rates fluctuated on millennial timescales in response to water-column ventilation rather than upstream oxidant demand in intermediate-depth waters. However, due to the poor age constraints available for Marine Isotopic Stage (MIS) 3, the EEP sedimentary data presented here could support two conflicting mechanisms, one driven by enhanced intermediate overturning circulation in the Southern Ocean during Heinrich Events/Antarctic Warm Events, implying that subsurface flow rates control thermocline ventilation, and a second one consistent with more sluggish intermediate circulation during Antarctic Warm Events and giving a central role to the temperature control on oxygen solubility in Southern Ocean surface waters.