Major shifts in nutrient and phytoplankton dynamics in the North Pacific Subtropical Gyre over the last 5000 years revealed by high-resolution proteinaceous deep-sea coral δ¹⁵N and δ¹³C records

Authors

Danielle S. Glynn
Kelton W. McMahon, University of Rhode IslandFollow
Thomas P. Guilderson
Matthew D. McCarthy

Document Type

Article

Date of Original Version

2019

Department

Oceanography

Abstract

The North Pacific Subtropical Gyre (NPSG) is the largest continuous ecosystem on Earth and is a critical component of global oceanic biogeochemical cycling and carbon sequestration. We report here multi-millennial-scale, sub-decadal-resolution records of bulk stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope records from proteinaceous deep-sea corals. Data from three Kulamanamana haumeaae specimens from the main Hawaiian Islands extend the coral-based time-series back ∼5000 yrs for the NPSG and bypass constraints of low resolution sediment cores in this oligotrophic ocean region. We interpret these records in terms of shifting biogeochemical cycles and plankton community structure, with a main goal of placing the extraordinarily rapid ecosystem biogeochemical changes documented by recent coral records during the Anthropocene in a context of broader Late-Holocene variability.

During intervals where new data overlaps with previous records, there is strong correspondence in isotope values, indicating that this older data represents a direct extension of Anthropocene records. These results reveal multiple large isotopic shifts in both δ¹⁵N and δ¹³C values similar to or larger in magnitude to those reported in the last 150 yrs. This shows that large fluctuations in the isotopic composition of export production in this region are not unique to the recent past, but have occurred multiple times through the Mid- to Late-Holocene. However, these earlier isotopic shifts occurred over much longer time intervals (∼millennial vs. decadal timescales). Further, the δ¹⁵N data confirm that the extremely low present day δ¹⁵N values recorded by deep sea corals (∼8‰) are unprecedented for the NPSG, at least within the past five millennia.

Together these records reveal centennial to millennial-scale oscillations in NPSG biogeochemical cycles. Further, these data also suggest a number of independent biogeochemical regimes during which δ¹⁵N and δ¹³C trends were synchronous (similar to recent coral records) or distinctly decoupled. We propose that phytoplankton species composition and nutrient source changes are the dominant mechanisms controlling the coupling and de-coupling of δ¹⁵N and δ¹³C values, likely primarily influenced by changing oceanographic conditions (e.g., stratification vs. entrainment). The decoupling observed in the past further suggests that oceanographic forcing and ecosystem responses controlling δ¹⁵N and δ¹³C values of export production have been substantially different earlier in the Holocene compared to mechanisms controlling the present day system.

Citation/Publisher Attribution

Glynn, D. S., McMahon, K. W., Guilderson, T. P., & McCarthy, M. D. (2019). Major shifts in nutrient and phytoplankton dynamics in the North Pacific Subtropical Gyre over the last 5000 years revealed by high-resolution proteinaceous deep-sea coral δ¹⁵N and δ¹³C records. Earth and Planetary Science Letters, 515, 145-153. doi:

Available at: https://doi.org/10.1016/j.epsl.2019.03.014