Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age

Abstract

The Southern Ocean regulates the ocean’s biological sequestration of CO2 and is widely suspected to underpin much of the ice age decline in atmospheric CO2 concentration, but the specific changes in the region are debated. Although more complete drawdown of surface nutrients by phytoplankton during the ice ages is supported by some sediment core-based measurements, the use of different proxies in different regions has precluded a unified view of Southern Ocean biogeochemical change. Here, we report measurements of the 15N/14N of fossil-bound organic matter in the stony deep-sea coral Desmophyllum dianthus, a tool for reconstructing surface ocean nutrient conditions. The central robust observation is of higher 15N/14N across the Southern Ocean during the Last Glacial Maximum (LGM), 18–25 thousand years ago. These data suggest a reduced summer surface nitrate concentration in both the Antarctic and Subantarctic Zones during the LGM, with little surface nitrate transport between them. After the ice age, the increase in Antarctic surface nitrate occurred through the deglaciation and continued in the Holocene. The rise in Subantarctic surface nitrate appears to have had both early deglacial and late deglacial/Holocene components, preliminarily attributed to the end of Subantarctic iron fertilization and increasing nitrate input from the surface Antarctic Zone, respectively

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

“…5). In contrast, nitrogen isotopic evidence has been interpreted as showing that the LGM Southern Ocean surface was almost completely devoid of NO 3 in summer (François et al 1997;Martínez-García et al 2014;Wang et al 2017), and the LGM deep ocean appears to have had significantly lower O 2 (Bradtmiller et al 2010;Galbraith and Jaccard 2015;Hoogakker et al 2015;Korff et al 2016) as expected given greater storage of respired carbon by the biological soft tissue pump. Recent evidence 1 3 even suggests that O 2 was quite low at 3 km depth in the LGM Southern Ocean (Jaccard et al 2016), and that this depletion may have extended to relatively shallow depths (Lu et al 2016).…”

“…One likely candidate is iron fertilization, which certainly contributed to Southern Ocean NO 3 drawdown to some degree (Martínez-García et al 2014), though proxy evidence for low Antarctic export production precludes this being the sole explanation of widespread NO 3 depletion Jaccard et al 2013;Wang et al 2017). Deepening of the remineralization profile, as would have resulted from slow ecosystem metabolism under low temperatures (Matsumoto et al 2007), could have contributed to low deep ocean oxygen concentrations, though again it seems unlikely to explain the surface nitrate depletion.…”

The isotope effect of nitrate assimilation in the Antarctic Zone: Improved estimates and paleoceanographic implications

“…During at least the last two ice ages, the d 15 N of diatomand deep-sea coral-bound organic N in Antarctic sediments was $4‰ higher than it is today (Studer et al, 2015;Wang et al, 2017), indicative of an enhanced degree of nitrate consumption during the ice ages. Together with the observed glacial decrease in Antarctic productivity (Kohfeld et al, 2005;Jaccard et al, 2013), this implies that the supply of NO 3 À to Antarctic Zone surface waters was significantly reduced during the ice ages, leading to the hypothesis of Antarctic ''stratification" as one of the dominant drivers of glacial-interglacial variation in atmospheric pCO 2 (Francois et al, 1997;Sigman et al, 2010). )…”

“…The isotopic fractionation of NO 3 À assimilation links the degree of NO 3 À consumption to the d 15 N (=(( 15 N/ 14 N) sample /( 15 N/ 14 N) reference À 1) Â 1000, with atmospheric N 2 as the reference) of both the NO 3 À and the newly produced organic matter (Altabet and Francois, 1994a,b;Sigman et al, 1999a). Thus, the d 15 N of fossil-bound organic matter recovered from Southern Ocean sediment cores provides a measure of the degree of nitrate consumption in the past and has been used to investigate possible mechanisms for driving the changes in atmospheric CO 2 observed over glacial cycles (Robinson and Sigman, 2008;Martinez-Garcia et al, 2014;Studer et al, 2015;Wang et al, 2017).…”

Natural forcing of the North Atlantic nitrogen cycle in the Anthropocene

“…A small portion of the coral's organic tissue is used to build the coral skeleton and is subsequently trapped in and protected by its mineral matrix (20). This protection preserves the organic matter against the diagenetic loss and exogenous N contamination that introduce uncertainty into non-fossil-bound archives of organic matter (21).…”