High-resolution Records of Proteinaceous Deep-Sea Coral δ13C and δ15N Values in the North Pacific Subtropical Gyre Suggest Major Shifts in Nutrient and Phytoplankton Dynamics over the last 5000 years

The North Pacific Subtropical Gyre (NPSG) is the largest continuous ecosystem on this planet, and currently expanding in a warming global climate. To understand current and future dynamics in productivity, biogeochemical cycling, and carbon sequestration, we must develop a more complete understanding of the dynamics in this important ecosystem in the past. Low sedimentation rates and high bioturbation make ocean-open sediment cores difficult to interpret at sufficiently high resolution. In contrast, deep-sea corals act as ‘living sediment traps’ and incorporate the signal of sinking organic matter directly into the chronological growth bands of their proteinaceous skeletons. We reconstructed a 5,000 year, high resolution (decadal-scale) record of past changes in stable bulk nitrogen (δ¹⁵N) and carbon isotopes (δ¹³C) from multiple deep-sea corals around the Hawaiian archipelago. Previous studies have indicated a substantial decrease in both δ¹⁵N and δ¹³C (1 to 1.5‰) since the onset of the Industrial Revolution (~1850s) to 1,000 year lows of 8‰ and -17‰ respectively (Sherwood et al. 2014, McMahon et al. 2015). Our new data now reveals that shifts of this magnitude are not unprecedented in the Mid- to Late Holocene. Our extended record indicates that over multi-millennial time scales there is a large range of natural variability, with δ¹⁵N values ranging from 8‰ to 12‰ and δ¹³C values ranging from -17‰ to -15‰. We propose that these signals reflect primarily shifts in phytoplankton species composition (as indicated by previous compound-specific work with amino acids). Comparisons with climate records suggest that these shifts may be directly linked to past changes in temperature (ocean stratification) and dust inputs. This study represents the first high-resolution record of nutrient and ecosystem dynamics in the NPSG over the past five millennia, and offers a historical baseline to better analyze the effects of current and future anthropogenic climate forcing.