Paleoproductivity and Stratification Across the Subarctic Pacific Over Glacial‐Interglacial Cycles

Abstract

In the Subarctic Pacific, variability in productivity on glacial‐interglacial timescales is often attributed to changes in stratification and nutrient delivery to the surface, but the mechanisms driving this relationship are poorly constrained. Records extending beyond the last glacial maximum from both the open ocean and the marginal seas are required to investigate the timing and magnitude of different influential processes through the full glacial cycle. In this study we generated 231Pa/230Th over 210,000 years in order to capture two full glacial cycles of paleoproductivity on the Juan de Fuca Ridge in the East Subarctic Pacific. The sedimentary 231Pa/230Th ratios are always equal to or greater than the seawater production ratio (0.093), consistent with enhanced biological scavenging in this region. The temporal pattern of 231Pa/230Th burial is remarkably coherent with changes in climate, with high values (0.20) during peak interglacial periods descending to low values (0.10) during peak glacial conditions, consistent with other long productivity records from this region. We investigate the possible contributions of temperature, sea ice formation, Bering Strait closure, wind strength, upwelling, and subsurface nutrient concentrations as possible mechanisms by which physical and/or chemical stratification emerged during glacial periods. Due to the low sea surface salinity in the North Pacific, cooling actually weakens the density gradient in surface (0–200 m) waters. To create the steep density profiles that characterize physical stratification, additional processes to reduce the salinity of surface waters must occur during glacial periods. We suggest that regional sea ice formation and Bering Strait closure may have contributed to freshening surface waters and enhancing physical stratification during glacial periods. Additionally, simulated weak winds in this region due to the southward shift of the glacial westerlies may have further reduced surface mixing depths in the Subarctic Pacific. Finally, previous model simulations suggest strong glacial wind stress curl in the Subarctic Pacific, but enhanced Ekman divergence of nutrient‐poor subsurface waters would have little impact on stimulating productivity in surface waters of the Subarctic Pacific. We therefore suggest that the combined effects of surface freshening, weak winds, and lower subsurface nutrient concentrations may all have contributed to lower productivity during glacial periods in the Subarctic Pacific.

Glacial-Interglacial Variations in Organic Carbon Burial in the Northwest Pacific Ocean Over the Last 380 kyr and its Environmental Implications

“…However, the distinct peaks of CaCO 3 and Ba bio contents (Figures 3B,D) observed during interglacials correlated with minima in OC fluxes, indicating paleo-productivity decoupled from OC burial over the past 380 kyr. Under the impact of the formation and enhancement of NPIW in the glacial stages, whose penetration depth hindered the supply of upward macronutrients from deeper water to the euphotic zone (Kohfeld and Chase, 2011;Worne et al, 2019), the observed lower paleo-productivity was similar to other glacial-interglacial cyclic records in the North Pacific (Jaccard et al, 2005;Li et al, 2017;Costa et al, 2018a). Therefore, in the Northwest Pacific, paleo-productivity during glacial periods with the driver of less macronutrient supply from deeper waters (Li et al, 2017) was contrary to that in high nitrate and low chlorophyll (HNLC) areas such as the subantarctic zone, where Fe derived from dust input was the main driver (Martinez-Garcia et al, 2014;Thöle et al, 2019).…”

ENSO vs glacial-interglacial-induced changes in the Kuroshio-Oyashio transition zone during the Pleistocene

“…However, the driving mechanisms for changes in productivity over longer time scales are not yet entirely understood. For example, to what extent the productivity was influenced by the complex interplay of atmosphere and ocean and their respective circulation patterns dependent on, e.g., sea ice formation, light limitation, wind strength, nutrient supply, and stratification (Haug and Sigman, 2009;Jaccard et al, 2010;Sigman et al, 2010;Sigman et al, 2021); Kawahata et al, 2000;Matsumoto et al, 2002;Brunelle et al, 2007;Shigemitsu et al, 2007;Jaccard et al, 2010;Nishioka et al, 2011;Costa et al, 2018;Sigman et al, 2021).…”

Population Biology of Fin Whales: Applying Demographic and Evolutionary Approaches to Studying Populations

“…Proxy data suggest comparatively lower levels of primary productivity at high latitudes during glaciations. In contrast, the rates of upwelling and cooler temperatures at lower latitudes resulted in an increase in primary productivity (Costa et al, 2018;Foster & Sexton, 2014;Hernández-Almeida et al, 2019;Radi & De Vernal, 2008;E. Thomas et al, 1995).…”

“…Thomas et al, 1995). The primary productivity increased at high latitudes, and decreased at lower latitudes when global temperatures increased after the LGM (Costa et al, 2018).…”

“…Anderson et al, 2002;Waelbroeck et al, 2009). The lower sea-surface temperature and increased sea-ice cover during the LGM affected ocean circulation and stratification (Costa et al, 2018). Proxy data suggest comparatively lower levels of primary productivity at high latitudes during glaciations.…”