Millennial scale oscillations in bulk δ15N and δ13C over the Mid- to Late Holocene seen in proteinaceous corals from the North Pacific Subtropical Gyre

Abstract:

The North Pacific Subtropical Gyre (NPSG) is the largest continuous ecosystem on this planet and is an important regulator of biogeochemical cycling and carbon sequestration. With evidence of its expansion in a warming climate, it is necessary to develop a more complete understanding of the variability in productivity and nutrient dynamics in this important ecosystem through time. We constructed a long-term, high resolution record of bulk record of stable nitrogen (δ¹⁵N) and carbon isotopes (δ¹³C) from multiple proteinaceous deep sea corals around Hawaii extending back ~5300 years with few gaps. Our data confirms the decreasing trend in δ¹⁵N since the Little Ice Age (1850s), which matches previously published results in part attributed to anthropogenic climate change (e.g. Sherwood et al. 2014). However, while the rate of change since the Little Ice Age (δ¹⁵N declines ~1‰ over ~150yrs) remains by far the most rapid throughout the longer record, there also appear to be longer-term (near-millennial scale) climatic oscillations of even greater magnitude (δ¹⁵N shifts ~1.5-2‰ over ~1000yrs). After removal of the Seuss Effect, δ¹³C values also declined ~1.5‰ since the Little Ice Age. Furthermore, there also appear to be oscillations in δ¹³C of ~1-2‰ over millennial timescales. These results reveal the existence of previously unrecognized long-term oscillations in NPSG biogeochemical cycles, which are likely linked to changes in phytoplankton species composition, food web dynamics, and/or variability in source nutrients and productivity possibly caused by changes in climate. This study provides insight into nutrient dynamics in the NPSG over the past five millennia, and offers a historical baseline to better analyze the effects of current anthropogenic climate forcing.