Similar Glacial‐Interglacial δ15N Variations in Two MIS 13–10 Sediment Sequences in the Western North Atlantic Ocean: Changes in Nitrogen Sources, Denitrification, or Diagenesis?

Abstract

AbstractWe have determined total organic carbon‐mass accumulation rates (TOC‐MARs) and δ15N values in Marine Isotope Stages (MIS) 13–10 sediments from two oligotrophic parts of the western North Atlantic Ocean—Ocean Drilling Program (ODP) Site 1063 on the Bermuda Rise and Site 1058 on the Blake Outer Ridge. Both TOC‐MARs and δ15N values vary significantly in these depositional records. TOC‐MARs are highest in sediments deposited at Site 1063 during MIS 12 and MIS 10, implying marine productivity or TOC preservation increased at this location during glacial stages, and low TOC‐MARs during most of MIS 11 in Site 1063 sediment imply low rates during this warm interval. In contrast to the Site 1063 record, productivity or preservation appears to have been greater at Site 1058 during parts of the MIS 11 interglacial than during the glacial intervals. Unlike the TOC‐MAR records, δ15N values at both locations exhibit similar glacial‐interglacial alternations. Values peak during substage 11.3 and gradually decrease toward the terminations of MIS 12 and MIS 10. The similarity of the δ15N patterns at these two sites implies that nitrogen cycling at these locations is largely independent of changes in local surface production of organic matter, and the higher δ15N values are unlikely to result from denitrification. Instead, the δ15N alternations likely result from glacial‐interglacial alternations between higher rates of sediment delivery to these western North Atlantic locations during glacial periods that favored TOC preservation and lower rates during interglacials that allowed more TOC degradation and associated diagenetic alteration of its nitrogen isotopic composition, leading to higher sediment δ15N values.