The influence of rapid, millennial scale climate change on nitrogen isotope dynamics of the Cariaco Basin during marine isotope stage 3

Abstract

Understanding changes to the marine nitrogen cycle on millennial and shorter time scales can help determine the influence of rapid climate change on the fixed N pool and its sources and sinks. Rapid changes in denitrification have been observed in the eastern tropical North Pacific (ETNP) and Arabian Sea; however, millennial scale δ15N records in regions influenced by N2 fixation are sparse. We present a sedimentary δ15N record from the Cariaco Basin during marine isotope stage (MIS) 3 (∼35-55 ka). The δ15N record displays a pattern of millennial scale variability that tracks the Greenland ice core Dansgaard-Oeschger cycles, with higher values observed during interstadial periods, lower values during stadial periods, and abrupt transitions in between. Conditions during interstadials are similar to those at present in the Cariaco Basin, with the sedimentary δ15N signal reflecting a combination of local processes and an imported regional signal. If interpreted to reflect regional processes, the interstadial δ15N values (average ∼5.1‰) support the argument that N2 fixation did not increase in the tropical North Atlantic during the last glacial. The lower δ15N values during stadials, when lower sea level resulted in increased physical isolation of the basin, can be explained primarily by local processes. In spite of the importance of local processes, striking similarity is observed between the Cariaco record and millennial scale δ15N records from the ETNP and Arabian Sea. The apparent synchronicity of changes observed in all three regions suggests an atmospheric teleconnection between the three sites and high-latitude climate forcing during MIS 3.