Janssen David J, Rickli Jörg, Wille Martin, Sepúlveda Steiner Oscar, Vogel Hendrik, Dellwig Olaf, Berg Jasmine S, Bouffard Damien, Lever Mark A, Hassler Christel S, Jaccard Samuel L
Institute of Geological Sciences University of Bern Bern Switzerland.
Oeschger Centre for Climate Change Research University of Bern Bern Switzerland.
Geophys Res Lett. 2022 Nov 16;49(21):e2022GL099154. doi: 10.1029/2022GL099154. Epub 2022 Oct 28.
Chromium stable isotope composition (δCr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δCr have not been assessed in modern redox-stratified basins. We present new chromium (Cr) concentration and δCr data in dissolved, sinking particulate, and sediment samples from the redox-stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non-quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox-stratified basins show analogous patterns. This challenges assumptions from δCr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non-quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δCr from oxidative continental weathering.
铬稳定同位素组成（δCr）是追溯地球历史上氧化还原条件的一种很有前景的示踪剂；然而，尚未在现代氧化还原分层盆地中评估δCr的地球化学控制因素。我们展示了来自氧化还原分层的卡达尼奥湖（瑞士）（一个现代元古代海洋模拟环境）的溶解态、下沉颗粒物和沉积物样品中的新的铬（Cr）浓度和δCr数据。这些数据表明，在化学跃层上方Cr发生不完全（非定量）还原和去除过程中存在同位素分馏现象，导致在缺氧深水中积累同位素较轻的Cr。沉积物自生Cr在同位素上与上覆水体不同，但与大陆地壳平均值相当。来自其他氧化还原分层盆地的新数据和已发表数据显示出类似模式。这对δCr古氧化还原应用中的假设提出了挑战，即定量的Cr还原和去除限制了同位素分馏。相反，非定量Cr去除导致的分馏使得沉积记录与上覆水体产生偏移，且无法反映来自氧化性大陆风化作用的高δCr值。
