Chromium concentration and isotopic composition of Cr(III) and Cr(VI) in the Eastern Tropical North Pacific from samples collected on R/V Roger Revelle and R/V Kilo Moana in April-May 2018 and Sept-Oct 2019

Understanding the cycling of chromium (Cr) and how chromium stable isotopes (δ53Cr) are altered in response to different processes in the modern ocean is important in our interpretation of marine sedimentary δ53Cr records, a promising redox proxy. Therefore, it is crucial to investigate the geochemical processes of Cr in reducing environments such as oxygen deficient zones (ODZs). In this study, we investigated the cycling of Cr in the Eastern Tropical North Pacific (ETNP) ODZ by analyzing the [Cr] and δ53Cr of total dissolved Cr and Cr(III). Our Cr(III) data at two inshore stations shows profile features and Cr reduction isotopic fractionation factor (-1.5‰) similar to an offshore station in a previous study. We also observed significant Cr scavenging signals in the upper 1000 meters (m) throughout the ODZ with an inshore-offshore variability in its magnitude. Specifically, anoxic bottom waters on the continental slope see the greatest Cr scavenging with heaviest δ53Cr (+1.85‰). Our estimates of the scavenged Cr isotopic composition are within error of the anoxic and euxinic marine sedimentary δ53Cr. This implies that the vertical transport of Cr to the seafloor and subsequent diagenesis may not generate significant isotopic fractionation for Cr. This is the first thorough investigation into the Cr cycling in the ETNP ODZ and demonstrated promising usage of marine sedimentary δ53Cr as a redox proxy for ancient oceans. In the ODZ, oxygen is consumed by degrading sinking particles and reaches extremely low levels (too low to support aerobic life) from 100m to 800m depth. However, microbes that can use other oxidants such as nitrate to metabolize organic carbon live there, and we showed that they also convert soluble anionic chromate Cr(VI) to cationic Cr(III), about half of which is scavenged onto sinking particles and removed to the seafloor. This reduction is accompanied by preferential reduction of light Cr isotopes, so the Cr(III) is 1.3‰ lighter than the source Cr(VI). The removal of part of this light Cr(III) by scavenging leaves the residual total Cr heavier than the source Cr. The analyzed samples listed here were chosen to be from the center and margins of the ETNP ODZ and over extremely reducing continental margin sediments.理解铬（Cr）的生物地球化学循环，以及现代海洋中不同过程如何影响铬稳定同位素（δ53Cr）的分馏特征，对于我们解读海洋沉积δ53Cr记录——这一极具潜力的氧化还原代用指标（redox proxy）——至关重要。因此，研究还原环境（如缺氧区（oxygen deficient zones，ODZs））内铬的地球化学过程极为关键。本研究以东热带北太平洋（Eastern Tropical North Pacific，ETNP）缺氧区（ETNP ODZ）为研究对象，通过分析总溶解态铬（total dissolved Cr）与三价铬（Cr(III)）的铬浓度（[Cr]）及δ53Cr值，探究了该区域的铬循环过程。本研究在两个近岸站位获取的Cr(III)数据显示，其剖面分布特征及铬还原过程的同位素分馏因子（isotopic fractionation factor，-1.5‰），均与此前一项研究中的远岸站位结果高度相似。同时，我们在整个缺氧区的1000米以浅水体中观测到显著的铬清除作用（scavenging）信号，且该信号的强度存在显著的近岸-远岸差异。具体而言，大陆坡（continental slope）上的缺氧底层水中铬清除作用最强，对应的δ53Cr值最高（+1.85‰）。我们估算的被清除铬的同位素组分，与缺氧与硫化海洋沉积δ53Cr（anoxic and euxinic marine sedimentary δ53Cr）的数值在误差范围内一致。这表明，铬向海底的垂直搬运及后续成岩作用（diagenesis）可能不会对铬产生显著的同位素分馏。本研究是首次对东热带北太平洋缺氧区的铬循环开展系统性深入研究，同时证实了海洋沉积δ53Cr作为古海洋氧化还原代用指标的应用潜力。 在缺氧区内，沉降颗粒（sinking particles）的降解过程会消耗水体中的溶解氧，使得100米至800米水深区间内的溶解氧浓度极低，低至无法维持需氧生物（aerobic life）的生存。然而，该区域栖息着可利用硝酸盐（nitrate）等其他氧化剂分解有机碳的微生物；本研究证实，这类微生物还可将可溶性阴离子铬酸盐Cr(VI)（soluble anionic chromate Cr(VI)）转化为阳离子态三价铬（Cr(III)，cationic Cr(III)），其中约一半会被吸附至沉降颗粒并被搬运至海底。该还原过程伴随轻铬同位素的优先还原（preferential reduction），因此生成的Cr(III)比初始Cr(VI)轻1.3‰。清除作用移除了部分轻同位素Cr(III)，使得残留的总溶解态铬比初始Cr(VI)更重。本文所列的分析样品均采自东热带北太平洋缺氧区的核心区域与边缘区域，以及极端还原的大陆边缘沉积物之上的水体。