Tracking Agulhas Leakage in the South Atlantic Using Modern Planktic Foraminifera Nitrogen Isotopes

IF 2.9 2区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS Geochemistry Geophysics Geosystems Pub Date : 2024-09-11 DOI:10.1029/2023GC011190
R. Granger, S. M. Smart, A. Foreman, A. Auderset, E. C. Campbell, T. A. Marshall, G. H. Haug, D. M. Sigman, A. Martínez-García, S. E. Fawcett
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Abstract

Seawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial-interglacial climate cycles. However, constraining past Agulhas leakage has been a challenge. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that the 15N/14N ratio (δ15N) of mixed-layer nitrate, zooplankton, and foraminifera (tissue and shells) was 2‰–3‰ lower in the eddy than in the background Atlantic even though the δ15N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ15N of foraminifera and other zooplankton in the eddy reflects the original Agulhas Current thermocline nitrate, which is ∼2‰ lower than that of the South Atlantic due to N2 fixation that occurs in the Indian Ocean. Foraminifera δ15N may have been lowered further during eddy migration by in situ N2 fixation and/or recycling of low-δ15N ammonium. The absence of low-δ15N Agulhas nitrate in the eddy thermocline can be explained by partial assimilation of the nitrate as it was mixed into the euphotic zone during and after eddy formation, raising its δ15N. The low δ15N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer-bound δ15N from the region could record variations in past Agulhas leakage.

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利用现代浮游有孔虫氮同位素追踪南大西洋的阿古哈斯漏水现象
通过 "阿古哈斯泄漏 "从印度洋输送到南大西洋的海水调节着全球海洋环流,并与冰川-间冰期气候周期有关。然而,制约过去的阿古哈斯泄漏一直是一个挑战。我们在2017年冬季对与成熟的阿古哈斯漩涡相交的开普盆地横断面进行了采样,发现混合层硝酸盐、浮游动物和有孔虫(组织和贝壳)的15N/14N比值(δ15N)在漩涡中比在大西洋背景中低2‰-3‰,尽管底层热层硝酸盐的δ15N在两种环境中没有区别。我们认为,漩涡中有孔虫和其它浮游动物的δ15N反映了阿古哈斯洋流热层硝酸盐的原始情况,由于印度洋中的N2固定作用,热层硝酸盐比南大西洋的低±2‰。有孔虫的δ15N可能在涡流迁移过程中通过原位固定N2和/或回收低δ15N铵而进一步降低。漩涡热层中没有低δ15N 的阿古尔哈斯硝酸盐,其原因可能是硝酸盐在漩涡形成期间和之后混入透光层时被部分同化,从而提高了其δ15N。即使在漩涡穿过开普盆地几个月后,漩涡有孔虫的δ15N仍然很低,这表明该地区有孔虫化石中的δ15N可以记录过去阿古哈斯泄漏的变化。
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来源期刊
Geochemistry Geophysics Geosystems
Geochemistry Geophysics Geosystems 地学-地球化学与地球物理
CiteScore
5.90
自引率
11.40%
发文量
252
审稿时长
1 months
期刊介绍: Geochemistry, Geophysics, Geosystems (G3) publishes research papers on Earth and planetary processes with a focus on understanding the Earth as a system. Observational, experimental, and theoretical investigations of the solid Earth, hydrosphere, atmosphere, biosphere, and solar system at all spatial and temporal scales are welcome. Articles should be of broad interest, and interdisciplinary approaches are encouraged. Areas of interest for this peer-reviewed journal include, but are not limited to: The physics and chemistry of the Earth, including its structure, composition, physical properties, dynamics, and evolution Principles and applications of geochemical proxies to studies of Earth history The physical properties, composition, and temporal evolution of the Earth''s major reservoirs and the coupling between them The dynamics of geochemical and biogeochemical cycles at all spatial and temporal scales Physical and cosmochemical constraints on the composition, origin, and evolution of the Earth and other terrestrial planets The chemistry and physics of solar system materials that are relevant to the formation, evolution, and current state of the Earth and the planets Advances in modeling, observation, and experimentation that are of widespread interest in the geosciences.
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