Distinguishing the Isotopic Signals of Nitrate Assimilation and Denitrification Along the GEOTRACES GP15 Pacific Meridional Transect

IF 3.4 2区 地球科学 Q1 OCEANOGRAPHY
Dario Marconi, Daniel M. Sigman, Karen L. Casciotti, Rian M. Lawrence, Wentao Wang, Sergey Oleynik, Alfredo Martínez-García
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Abstract

The biogeochemical processes shaping the fluxes of nitrogen (N) in the tropical to subarctic Pacific are illuminated by nitrate isotope ratios (δ15N and δ18O) along the GEOTRACES GP15 section. In the equatorial and tropical Pacific, nitrate δ15N and δ18O are 2–4‰ higher in the thermocline than in deeper waters. This widespread elevation is driven by both nitrate assimilation in Southern Ocean surface waters and denitrification in the eastern tropical Pacific. In addition to this, a poleward increase in the δ15N of surface nitrate from the Equator to 10°S is generated by the progressive consumption of nitrate upwelled at the Equator and transported southward. This process leads to increases in the δ15N of phytoplankton biomass and a strong meridional nitrate isotopic gradient in the thermocline due to regeneration of sinking N. North of the Equator, an analogous gradient is barely detectable due to weaker northward flow and a compressed spatial scale for nitrate drawdown. By contrast, between 5°N and 40°N, the nitrate isotope gradients are dominated by the processes driving the oceanic fixed N budget: denitrification and N2 fixation. High-δ15N and -δ18O nitrate in the tropics is advected from the oxygen deficient zone where denitrification occurs, whereas in the adjacent subtropics, low-δ15N thermocline nitrate suggests a response by N2 fixation that is not observed in the south. The asymmetry of the north and south tropical Pacific gradients in thermocline nitrate isotopes has implications for efforts to reconstruct the N cycle in the past.

Abstract Image

GEOTRACES GP15太平洋经向样带硝酸盐同化和反硝化的同位素信号识别
GEOTRACES GP15剖面的硝酸盐同位素比值(δ15N和δ18O)揭示了形成热带至亚北极太平洋氮(N)通量的生物地球化学过程。在赤道和热带太平洋,温跃层中硝酸盐δ15N和δ18O比深海中高2 ~ 4‰。这种广泛的海拔升高是由南大洋表层水的硝酸盐同化和热带太平洋东部的反硝化作用共同驱动的。此外,从赤道到10°S,地表硝酸盐的δ15N向极地方向增加是由赤道上涌的硝酸盐逐渐消耗并向南输送造成的。这一过程导致浮游植物生物量的δ15N增加,并且由于下沉的n的再生,在赤道以北,由于较弱的北流和压缩的硝酸盐下降的空间尺度,类似的梯度几乎无法检测到。相反,在5°N ~ 40°N之间,硝酸盐同位素梯度主要由驱动海洋固定氮收支的过程主导:反硝化和固氮。热带地区的高δ 15n和-δ18O硝酸盐是从发生反硝化作用的缺氧区平流过来的,而在邻近的亚热带地区,低δ 15n温跃层硝酸盐表明了对N2固定的响应,这在南方没有观察到。温跃层硝酸盐同位素中南北热带太平洋梯度的不对称性对重建过去的N循环具有重要意义。
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来源期刊
Journal of Geophysical Research-Oceans
Journal of Geophysical Research-Oceans Earth and Planetary Sciences-Oceanography
CiteScore
7.00
自引率
13.90%
发文量
429
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