Investigating the source of orbital-scale δ15N variability in Western Equatorial Pacific sediments through MIS-11
Abstract
The Western Pacific Warm Pool (WPWP) region is an oceanic crossroads where many currents and water masses in the Pacific begin, diverge, and terminate, contributing to the complex nutrient budget of the region. In 2016, International Ocean Discovery Program Expedition 363 drilled sites U1486 and U1487 in the WPWP approximately 160 km from northern New Guinea. The sites are located 25 km apart on the seaward side of a sedimentary trough isolating them from significant terrigenous input. They are also directly in the path of the South Pacific western boundary current system. To better understand past variability in organic matter flux to the sediments, δ15N was measured on bulk sediments at both sites back to MIS-11 (450 kyr). Initial age models were informed by planktonic foraminifera δ18O and subsequently calibrated to benthic δ18O. Bulk sediment δ15N covaries on orbital timescales at both sites, with substantial power in the precession band (21 kyr). Downcore δ15N at U1486 ranges from 7.2 to 13.9‰, with U1487 ranging between 8.2 and 13.3‰ - both well above the deep ocean mean of 5‰. Highest δ15N occurs during early interglacials and lowest during glacials, with observed precessional variability correlating strongest to 65°N summer insolation. At other Western Pacific sites, sediment δ15N has been linked to orbital-scale changes in Eastern Equatorial Pacific (EEP) water column denitrification (WCD) as well as enrichment via NO3- utilization along the westward flow path. Contemporary δ15N of NO3- in the Central Pacific Line Islands has been measured up to 19‰, decreasing westward primarily due to dilution of the EEP δ15N signal by nitrogen fixation and water mass mixing as well as possible terrigenous influences in the WPWP. During periods of relative global warmth, δ15N at the two sites is highest due to potential increased WCD and NO3- utilization along the flow path from the EEP, with minimal dilution. During cold periods, δ15N is at a minimum due to potential decreased WCD and nitrate utilization, with increased dilution. Orbital-scale changes in South Pacific gyre circulation contributing to variability in the South Equatorial Current and advection of Antarctic Intermediate Water into the tropical thermocline may be mechanisms for propagation and dilution of the upstream δ15N signal to the sites throughout the record.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP51C1385L
- Keywords:
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- 1616 Climate variability;
- GLOBAL CHANGE;
- 1631 Land/atmosphere interactions;
- GLOBAL CHANGE;
- 3022 Marine sediments: processes and transport;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3036 Ocean drilling;
- MARINE GEOLOGY AND GEOPHYSICS