Surface nitrate utilization in the Bering sea since 180 kA BP: Insight from sedimentary nitrogen isotopes

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Riethdorf, Jan-rainer, Thibodeau, Benoit, Ikehara, Minoru, Nuernberg, Dirk, Max, Lars, Tiedemann, Ralf, Yokoyama, Yusuke
• Format: Text
• Language: English
• Published: Pergamon-elsevier Science Ltd
• Subjects: envir; geo; Bering Sea; Greenland
• Online Access: https://doi.org/10.1016/j.dsr2.2015.03.007; https://archimer.ifremer.fr/doc/00421/53261/83376.pdf; https://archimer.ifremer.fr/doc/00421/53261/

We present high-resolution records of sedimentary nitrogen (delta N-15(bulk)) and carbon isotope ratios (delta C-13(bulk)) from piston core S0201-2-85KL located in the western Bering Sea. The records reflect changes in surface nitrate utilization and terrestrial organic matter contribution in submillennial resolution that span the last 180 kyr. The delta N-15(bulk) record is characterized by a minimum during the penultimate interglacial indicating low nitrate utilization (similar to 62-80%) despite the relatively high export production inferred from opal concentrations along with a significant reduction in the terrestrial organic matter fraction (m(terr)). This suggests that the consumption of the nitrate pool at our site was incomplete and even more reduced than today (similar to 84%). delta N-15(bulk) increases from Marine Isotope Stage (MIS) 5.4 and culminates during the Last Glacial Maximum, which indicates that nitrate utilization in the Bering Sea was raised during cold intervals (MIS 5.4, 5.2, 4) and almost complete during MIS 3 and 2 (similar to 93-100%). This is in agreement with previous hypotheses suggesting that stronger glacial stratification reduced the nutrient supply from the subeuphotic zone, thereby increasing the iron-to-nutrient ratio and therefore the nitrate utilization in the mixed surface layer. Large variations in delta N-15(bulk) were also recorded from 180 to 130 ka BP (MIS 6), indicating a potential link to insolation and sea-level forcing and its related feedbacks. Millennial-scale oscillations were observed in delta N-15(bulk) and delta C-13(bulk) that might be related to Greenland interstadials.