Abstract
The modern North Pacific plays a critical role in marine biogeochemical cycles, as an oceanic sink of CO2 and by bearing some of the most productive and least oxygenated waters of the World Ocean. The capacity to sequester CO2 is limited by efficient nutrient supply to the mixed layer, particularly from deeper water masses in the Pacific's subarctic and marginal seas. The region is in addition only weakly ventilated by North Pacific Intermediate Water (NPIW), which receives its characteristics from Okhotsk Sea Intermediate Water (OSIW). Here, we present reconstructions of intermediate water ventilation and productivity variations in the Okhotsk Sea that cover the last glacial termination between eight and 18 ka, based on a set of high-resolution sediment cores from sites directly downstream of OSIW formation. In a multi-proxy approach, we use total organic carbon (TOC), chlorin, biogenic opal, and CaCO3 concentrations as indicators for biological productivity. C/N ratios and XRF scanning-derived elemental ratios (Si/K and Fe/K), as well as chlorophycean algae counts document changes in Amur freshwater and sediment discharge that condition the OSIW. Stable carbon isotopes of epi- and shallow endobenthic foraminifera, in combination with 14C analyses of benthic and planktic foraminifera imply decreases in OSIW oxygenation during deglacial warm phases from c. 14.7 to 13 ka (Bølling-Allerød) and c. 11.4 to 9 ka (Preboreal). No concomitant decreases in Okhotsk Sea benthic-planktic ventilation ages are observed, in contrast to nearby, but southerly locations on the Japan continental margin. We attribute Okhotsk Sea mid-depth oxygenation decreases in times of enhanced organic matter supply to maxima in remineralization within OSIW, in line with multi-proxy evidence for maxima in primary productivity and supply of organic matter. Sedimentary C/N and Fe/K ratios indicate more effective entrainment of nutrients into OSIW and thus an increased nutrient load of OSIW during deglacial warm periods. Correlation of palynological and sedimentological evidence from our sites with hinterland reference records suggests that millennial-scale changes in OSIW oxygen and nutrient concentrations were largely influenced by fluvial freshwater runoff maxima from the Amur, caused by a deglacial northeastward propagation of the East Asian Summer Monsoon that increased precipitation and temperatures, in conjunction with melting of permafrost in the Amur catchment area. We suggest that OSIW ventilation minima and the high lateral supply of nutrients and organic matter during the Allerød and Preboreal are mechanistically linked to concurrent maxima in nutrient utilization and biological productivity in the subpolar Northwest Pacific. In this scenario, increased export of nutrients from the Okhotsk Sea during deglacial warm phases supported subarctic Pacific shifts from generally Fe-limiting conditions to transient nutrient-replete regimes through enhanced advection of mid-depth nutrient- and Fe-rich OSIW into the upper ocean. This mechanism may have moderated the role of the subarctic Pacific in the deglacial CO2 rise on millennial timescales by combining the upwelling of old carbon-rich waters with a transient delivery of mid-depth-derived bio-available Fe and silicate.
Highlights
Today, no new deepwater is formed in the subarctic North Pacific, because the surface water masses are not dense enough to sink to significant water depths and initiate convection due to low surface salinities and an associated strong halocline (EmileGeay et al, 2003; Warren, 1983)
We carried out a comprehensive multi-proxy-based reconstruction of Okhotsk Sea Intermediate Water ventilation and biogeochemical characteristics over the last glacial termination based on a set of high-resolution AMS 14C-dated sediment cores
Based on ventilation ages and epibenthic stable carbon isotopes we have provided evidence that decreases in Okhotsk Sea Intermediate Water (OSIW) ventilation were largely driven by increases in organic matter content and remineralization rates, rather than changes in formation rates and overturning of OSIW in the Okhotsk Sea
Summary
No new deepwater is formed in the subarctic North Pacific, because the surface water masses are not dense enough to sink to significant water depths and initiate convection due to low surface salinities and an associated strong halocline (EmileGeay et al, 2003; Warren, 1983). Modern NPIW and OSIW are highly variable in their biogeochemical characteristics, even on relatively short (i.e. multidecadal) instrumental timescales (Emerson et al, 2004; Tadokoro et al, 2009), and keep mid-depth waters in the North Pacific moderately oxygenated. While the North Pacific overall is a large modern oceanic sink for atmospheric CO2 (Takahashi et al, 2009), surface utilization of nutrients by primary producers and export production of carbon (the “biological pump”) remains incomplete in the Western Subarctic Pacific (WSAP) Gyre, mainly due to the rapid seasonal depletion of iron (Fe) as micronutrient (see review by Takeda, 2011), and silicate (Si(OH)4; Harrison et al, 2004). Galbraith et al, 2008; Jaccard et al, 2005) This more effective utilization allowed a lower primary production to export carbon from the surface into the deep ocean more efficiently. Indicators for export production varied in phase with atmospheric CO2 concentrations for at least the last 800 ka (Jaccard et al, 2010)
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