A Test of the Diatom‐Bound Paleoproxy: Tracing the Isotopic Composition of Nutrient‐Nitrogen Into Southern Ocean Particles and Sediments

Abstract

Sedimentary nitrogen isotope (as δ 15 N) records from the Southern Ocean provide critical constraints on surface nutrient consumption in the past and the role of Southern Ocean biophysical changes in setting atmospheric pCO 2. We present a field assessment of how surface nitrate consumption is reflected in δ 15 N values of total nitrogen and diatom-bound nitrogen pools of particles and sediments across the Southern Ocean along 170°W during late austral summer. Mixed layer nitrate δ 15 N values increase northwards associated with greater nitrate drawdown. Particles and sediments are expected to follow this trend. Contrary to expectations, surface ocean particle total nitrogen and diatom-bound δ 15 N values decreased northward during the late summer, likely due to recycling of nitrogen and the assimilation of regenerated ammonium, as well as nitrate. The relationship between δ 15 N values of the total nitrogen and diatom-bound pools remains relatively constant across this Southern Ocean transect, suggesting that the isotopic composition of these two surface ocean nitrogen pools are largely set by the δ 15 N value(s) of the assimilated nutrient(s). Surface sediment δ 15 N values do increase away from the region of maximum biogenic silica deposition, suggesting that the recycled nitrogen isotopic signal observed in late summer particles may not significantly impact the sedimentary record. However, the enrichment in δ 15 N values of the diatom-bound pool is greater than what is expected from progressive utilization of the surface nitrate alone and not yet explained. Plain Language Summary Southern Ocean biology helps to draw carbon dioxide out of the atmosphere. Here, we present a test of how well the diatom-bound nitrogen isotope paleoproxy records past surface ocean nutrient conditions, a critical constraint on the role of this biological carbon pump. Overall, the diatom nitrogen pool records the isotopic composition of their source of nutrients. Contrary to expectations, the nitrogen isotopes of the surface ocean particles showed a northward decrease across the Southern Ocean rather than the expected northward increase estimated from the nitrate pool. This is likely due to nitrate and ammonium uptake during the summer rather than growth on nitrate alone. The sedimentary nitrogen pools on the other hand showed an isotope trend more aligned with expectation, but in the case of the diatom-bound nitrogen, with a steeper gradient. This difference between the water column and the sediments likely stems from sampling during the summer, after the bloom and main sedimentation event, and it suggests that the recycled signal reflects a minor contribution to the overall sedimentary record.

Biogeochemical controls on wintertime ammonium accumulation in the surface layer of the Southern Ocean

“…Palaeoceanographic proxies -NH₄⁺ cycling in the Southern Ocean mixed layer may be important for palaeoceanographic proxies (Smart et al, 2020;Robinson et al, 2020), such as those that use the 15 N of organic matter preserved in fossil foraminifer or diatom shells to infer the extent of upper ocean NO 3 consumption in the past (and by extension, the role of Southern Ocean biology in determining atmospheric CO 2 ; e.g., Martínez-García et al, 2014;Studer et al, 2015). A recent ground-truthing study from the Southern Ocean showed that the 15 N of foraminifer-bound organic N tracks the 15 N of PON rather than NO 3 - (Smart et al, 2020), in contrast to results from the low-latitude ocean (Ren et al, 2012;Smart et al, 2015).…”

Changes in isotope fractionation during nitrate assimilation by marine eukaryotic and prokaryotic algae under different pH and CO2 conditions

“…Nitrogen (N) isotopic compositions of oceanic nitrate and bulk sediments are powerful indicators for evaluating surface nitrate utilization in the modern and past oceans, facilitating the understanding of spatial and temporal changes in the strength of the biological pump (Altabet and Francois 1994; Robinson et al 2020). The key to the application of stable N isotope tools is to understand the magnitude of the isotope effect (i.e., a phenomenon associated with the isotopes of a given element exhibiting different rates during reactions, which is reported as ε = ( light k / heavy k – 1) × 1000, expressed in ‰, in which k refers to the specific reaction rate constants of light and heavy isotopes) for nitrate assimilation.…”