Deglacial development of (sub) sea surface temperature and salinity in the subarctic northwest Pacific: Implications for upper‐ocean stratification

Abstract

[1] Based on models and proxy data, it has been proposed that salinity-driven stratification weakened in the subarctic North Pacific during the last deglaciation, which potentially contributed to the deglacial rise in atmospheric carbon dioxide. We present high-resolution subsurface temperature (T Mg/Ca ) and subsurface salinity-approximating (d 18O ivc-sw ) records across the last 20,000 years from the subarctic North Pacific and its marginal seas, derived from combined stable oxygen isotopes and Mg/Ca ratios of the planktonic foraminiferal species Neogloboquadrina pachyderma (sin.). Our results indicate regionally differing changes of subsurface conditions. During the Heinrich Stadial 1 and the Younger Dryas cold phases, our sites were subject to reduced thermal stratification, brine rejection due to sea-ice formation, and increased advection of low-salinity water from the Alaskan Stream. In contrast, the Bølling-Allerød warm phase was characterized by strengthened thermal stratification, stronger sea-ice melting, and influence of surface waters that were less diluted by the Alaskan Stream. From direct comparison with alkenone-based sea surface temperature estimates (SST Uk 0 37 ), we suggest deglacial thermocline changes that were closely related to changes in seasonal contrasts and stratification of the mixed layer. The modern upper-ocean conditions seem to have developed only since the early Holocene.

Millennial-scale variability of marine productivity and terrigenous matter supply in the western Bering Sea over the past 180 kyr

“…Deglacial maxima of CaCO 3 in Bering Sea sediments were also explained by denitrification on continental shelves, which might have resulted in an increase in alkalinity and, thus, in enhanced carbonate preservation (Chen, 2002;Okazaki et al, 2005). Today, the calcite saturation horizon in the Bering Sea is reported to lie above 500 m water depth (Feely et al, 2002) and at our sites lies above 200 m water depth (Riethdorf et al, 2013). Accordingly, we consider CaCO 3 maxima in our cores to mainly reflect a higher bottom water calcite saturation state, but enhanced biological CaCO 3 production can not be ruled out.…”

“…Increasing insolation should have amplified the surface ocean warming and led to more dynamic ice conditions, northward propagating ice margins, and a prolonged sea-ice-free summer season. Indeed, recent alkenone-and Mg/Ca-based studies indicate rising SST, strengthened thermal mixed layer stratification (stronger seasonal contrasts), and the onset of coccolithophorid production during the B/A (Caissie et al, 2010;Max et al, 2012;Riethdorf et al, 2013). Enhanced surface freshening due to melting sea ice is supported by higher abundances of radiolarian species Rhizoplegma boreale (Kim et al, 2011), as well as brackish diatom species Paralia sulcata (Gorbarenko et al, 2005).…”

Combined oxygen and silicon isotope analysis of diatom silica from a deglacial subarctic Pacific record

“…The observed changes in nutrient utilization and productivity are probably linked to the finding of Riethdorf et al . () suggesting an enhancement of the thermal mixed‐layer stratification and related increase in seasonal temperature contrasts, comparable to modern conditions, developing since the Early Holocene.…”

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

“…When sea ice forms, it fractionates δ 18 O such that the ice is isotopically enriched and the residual brine is isotopically depleted (O'Neil, ), making sea ice formation the only process by which a negative correlation between δ 18 O and salinity develops (Hillaire‐Marcel & de Vernal, ). Thus, sea ice melt can be identified in surface waters (<50 m) by the presence of cold (fresh) and high δ 18 O sw water overlying saltier but lower δ 18 O sw water (subducted brines; Brennan et al, ; Hillaire‐Marcel & de Vernal, ; Riethdorf et al, ). Such a profile may be constructed by analyzing paired δ 18 O and temperature (e.g., Mg/Ca) in planktonic foraminifera that inhabit different depths in the surface ocean (Figure ; e.g., Kumar et al, ; Parker et al, ; Wara et al, ).…”