Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean

Abstract

The interplay between ocean circulation and biological productivity affects atmospheric CO2 levels and marine oxygen concentrations. During the warming of the last deglaciation, the North Pacific experienced a peak in productivity and widespread hypoxia, with changes in circulation, iron supply, and light limitation all proposed as potential drivers. Here we use the boron-isotope composition of planktic foraminifera from a sediment core in the western North Pacific to reconstruct pH and dissolved CO2 concentrations from 24,000 to 8,000 years ago. We find that the productivity peak during the Bølling-Allerød warm interval, 14,700 to 12,900 years ago, was associated with a 2 decrease in near-surface pH and an increase in pCO2, and must therefore have been driven by increased supply of nutrient and CO2-rich waters. In a climate model ensemble (PMIP3), the presence of large ice sheets over North America results in high rates of wind-driven upwelling within the subpolar North Pacific. We suggest that this process, combined with collapse of North Pacific Intermediate Water formation at the onset of the Bølling-Allerød, led to high rates of upwelling of water rich in nutrients and CO2, and supported the peak in productivity. The respiration of this organic matter, along with poor ventilation, likely caused the regional hypoxia. We suggest that CO2 outgassing from the North Pacific helped to maintain high atmospheric CO2 concentrations during the Bølling-Allerød and contributed to the deglacial CO2 rise. The balance between the supply of nutrients and carbon to surface waters via ocean circulation and their removal via biological productivity is a first order control on atmospheric CO2. This is particularly important at high latitudes 1,2 , where Ekman suction, driven by cyclonic wind stress, and winter mixing supply CO2 from the deep ocean to the surface. Depending on the availability of light and micronutrients, such as iron, this CO2 may be captured by photosynthesis and returned to the deep ocean (the "biological pump"), or may escape to the atmosphere. The balance between circulation and biological productivity is also crucial for determining the ocean's dissolved oxygen content. Ventilation of the ocean at high latitudes mixes oxygen-rich waters from the surface into the ocean's interior, where oxygen is consumed

Wind‐Driven Evolution of the North Pacific Subpolar Gyre Over the Last Deglaciation

“…A more tilted jet stream does not seem unreasonable given the large changes in the size of the North American ice sheets beginning at this time (e.g., Lambeck et al, 2014) and is in good agreement with terrestrial proxy records and paleoclimatic simulations of this time period (Lora et al, 2016;Wong et al, 2016). Increased heat transport from a more tilted gyre could help explain the anomalous warmth of the SPG during the Bølling-Allerød (e.g., Gray et al, 2018) and may help Figure S8 for meridional profiles of SST, barotropic stream function, and zonal wind stress. drive wider Northern Hemisphere warming and ice sheet collapse at this time.…”

“…A more tilted jet stream does not seem unreasonable given the large changes in the size of the North American ice sheets beginning at this time (e.g., Lambeck et al, ) and is in good agreement with terrestrial proxy records and paleoclimatic simulations of this time period (Lora et al, ; Wong et al, ). Increased heat transport from a more tilted gyre could help explain the anomalous warmth of the SPG during the Bølling‐Allerød (e.g., Gray et al, ) and may help drive wider Northern Hemisphere warming and ice sheet collapse at this time. We note that the tilt of the gyre in the modern North Atlantic is poorly simulated by climate models (Zappa et al, ), and thus, it may also be poorly simulated in the North Pacific.…”

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

“…During glacial periods, the wind stress curl would have increased in the Subarctic as the westerlies, the zero point of wind stress curl (Thomson, ), shifted southward and away from the Subarctic (Figure ). Indeed, models suggest that wind stress curl may have been as much as 60% higher in this region during the last glacial period (Gray et al, ), increasing Ekman divergence and shoaling sub‐surface waters. Yet glacial productivity was still lower than interglacial productivity.…”

“…Nutrient concentrations within subsurface waters (>150 m) are a balance between mixing of young, nutrient‐poor NPIW formed in the Okhotsk Sea (Talley, ; You, ) and old, nutrient‐rich North Pacific Deep Water, and modern NPIW is characterized by a steep vertical seawater δ 13 C gradient from high δ 13 C at the surface to low δ 13 C at depth (Kroopnick, ). Enhanced NPIW formation during glacial periods (see section 4.2.1) expanded the high δ 13 C (low nutrient) water mass to 2,000 m (Keigwin, ; Matsumoto et al, ), pushing the nutrient‐rich North Pacific Deep Water to greater depths (Boyle, ), and reducing the nutrient concentrations of subsurface waters potentially attainable by winter mixing (Gray et al, ). Thus, lower subsurface nutrient concentrations likely combined with weaker winds and enhanced surface freshening to reduce productivity during glacial periods.…”

Atmosphere‐Ocean CO2 Exchange Across the Last Deglaciation From the Boron Isotope Proxy

“…By taking advantage of the calibrated boron‐pH proxy, a number of studies have investigated the history of ocean‐atmosphere CO 2 exchange across the last deglaciation from sites in the tropical Pacific (Martínez‐Botí et al, ; Palmer & Pearson, ), the North Pacific (Gray, Rae, et al, ), the North Indian Ocean (Naik et al, ; Palmer et al, ), the tropical Atlantic (Foster, ; Foster & Sexton, ; Henehan et al, ), the North Atlantic (Ezat et al, ; Yu et al, ), and the South Atlantic (Martínez‐Botí et al, ; Figure a). Collectively, the available records reveal millennial‐scale δ 11 B variations that reflect substantial changes in air‐sea ∆pCO 2 over the deglaciation (Figure ).…”