Impact of glacial/interglacial sea level change on the ocean nitrogen cycle

Abstract

The continental shelves are the most biologically dynamic regions of the ocean, and they are extensive worldwide, especially in the western North Pacific. Their area has varied dramatically over the glacial/interglacial cycles of the last million years, but the effects of this variation on ocean biological and chemical processes remain poorly understood. Conversion of nitrate to N by denitrification in sediments accounts for half or more of the removal of biologically available nitrogen ("fixed N") from the ocean. The emergence of continental shelves during ice ages and their flooding during interglacials have been hypothesized to drive changes in sedimentary denitrification. Denitrification leads to the occurrence of phosphorus-bearing, N-depleted surface waters, which encourages N fixation, the dominant N input to the ocean. An 860,000-y record of foraminifera shell-bound N isotopes from the South China Sea indicates that N fixation covaried with sea level. The N fixation changes are best explained as a response to changes in regional excess phosphorus supply due to sea level-driven variations in shallow sediment denitrification associated with the cyclic drowning and emergence of the continental shelves. This hypothesis is consistent with a glacial ocean that hosted globally lower rates of fixed N input and loss and a longer residence time for oceanic fixed N-a "sluggish" ocean N budget during ice ages. In addition, this work provides a clear sign of sea level-driven glacial/interglacial oscillations in biogeochemical fluxes at and near the ocean margins, with implications for coastal organisms and ecosystems.

Nitrogen Fixation Changes Regulated by Upper Water Structure in the South China Sea During the Last Two Glacial Cycles

“…After removing the glacial‐interglacial cycle trend, the detrended δ 15 N Src (δ 15 N Src‐residue ) displays a clear precession cycle paralleled to the δ 15 N bulk record, especially during MIS5 and MIS7 (Figure d). The precession cycle is also extensively found in FB‐δ 15 N record in the eastern SCS (Ren et al, ) and δ 15 N org records in the southern SCS (Dong et al, ; results of spectral analyses are shown in the supporting information). Previous study by Ren et al () suggest that the N fixation changes of the SCS are mainly controlled by sea level‐driven glacial/interglacial oscillations in excess phosphorous fluxes at and near the ocean margins.…”

“…The δ 15 N records from these three sites all present glacial‐interglacial variations of ~3‰, with distinct lower values in interglacial periods. The FB‐δ 15 N is supposed to faithfully represent the subsurface nitrate δ 15 N (Ren et al, ), as the late Holocene FB‐δ 15 N in SCS cores average 5.60‰ (Ren et al, ), close to the modern subsurface nitrate δ 15 N (Figure a). The MD05‐2901 δ 15 N Src varies in the same pattern and amplitude as those FB‐δ 15 N over the last 250 ka but shows a systematic offset of about +1‰ (Figure c).…”

“…Proxy records of the South China Sea (SCS) δ 15 N and upper water thermal structure. (a) FB‐δ 15 N from MD12‐3432 (light green) and MD97‐2142 (dark green; Ren et al, ; Wang et al, ). (b) Organic δ 15 N (δ 15 N org ) from MD05‐2897 (blue) and MD01‐2392 (dark purple; Dong et al, ).…”

“…δ 15 N bulk from the adjacent SCS are relatively low with no systematic correlation to climatic cycles (Kienast, ). In contrast, recently published foraminifera‐bound δ 15 N (FB‐δ 15 N) records in the SCS show distinct lower FB‐δ 15 N during interglacials (Ren et al, ; Ren et al, ; Wang et al, ). The disagreement between δ 15 N bulk and FB‐δ 15 N is very likely due to influences of terrestrial inputs and diagenetic effects.…”

“…The vertical δ 15 N pattern along with the nitrate concentration profiles indicates that primary production in the surface water is mainly supported by subsurface nitrate. The δ 15 N nitrate minimum in the subsurface is also an evidence of surface N fixation: Remineralization of 15 N‐poor organic matter formed by nitrogen fixation causes the δ 15 N nitrate minimum in the upper thermocline (Ren et al, ; Wong et al, ).…”