Progressive ocean oxygenation at ~2.2 Ga inferred from geochemistry and molybdenum isotopes of the Nsuta Mn deposit, Ghana
Progressive ocean oxygenation at ~2.2 Ga inferred from geochemistry and molybdenum isotopes of the Nsuta Mn deposit, Ghana
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DOI：
10.1016/j.chemgeo.2021.120116
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发表时间：
2021-04
期刊：
影响因子：
3.9
通讯作者：
K. Goto;Y. Sekine;Takashi Ito;Katsuhiko Suzuki;A. Anbar;G. Gordon;Y. Harigane;T. Maruoka;G. Shimoda;T. Kashiwabara;Y. Takaya;T. Nozaki;J. Hein;G. M. Tetteh;F. Nyame;S. Kiyokawa
中科院分区：
文献类型：
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作者：
K. Goto;Y. Sekine;Takashi Ito;Katsuhiko Suzuki;A. Anbar;G. Gordon;Y. Harigane;T. Maruoka;G. Shimoda;T. Kashiwabara;Y. Takaya;T. Nozaki;J. Hein;G. M. Tetteh;F. Nyame;S. Kiyokawa
最近的地球化学数据表明，在中-古元古代（~2.3-2.0 Ga）的O2超冲的发生。这种O2超调似乎与碳同位素记录相一致，表明在此期间有机碳的高埋藏率，即所谓的Lomagundi事件。然而，鲜为人知的是，在海洋氧化还原条件的变化与O2过冲。为了更好地了解中-古元古代海洋化学，我们研究了加纳Birimian超群Nsuta锰存款中的锰矿石和千枚岩样品的显微结构、主元素和微量元素浓度、Re-Os和Mo（δ98/95 Mo）同位素以及总有机碳含量。锰矿样品含有早成岩菱锰矿（碳酸锰）。锰矿石样品的微量元素组成和Re-Os同位素表明，菱锰矿来源于2.2Ga沉积的原生锰氧化物（MnO 2）。蚀变最小的锰矿样品的δ98/95 Mo值介于−1.10‰和−0.55‰之间（相对于NIST 3134），表明在O2超冲期间海水δ98/95 Mo值为1.85 ± 0.18‰（1 SD）。这种高的海水δ98/95 Mo值可以通过锰氧化物吸附增强同位素轻Mo的去除来最好地解释。为了形成广泛的锰氧化物矿床，O2浓度> 10 μM的底层海水在~2.2 Ga时发生了膨胀。古元古代中期的氧化条件可能支持了茎群真核生物的出现。
Recent geochemical data suggest the occurrence of an O2overshoot during the mid-Paleoproterozoic (~2.3–2.0 Ga). This O2overshoot appears to be consistent with carbon isotope records that suggest high burial rates of organic carbon during that period, the so-called Lomagundi Event. However, little is known about the changes in the ocean redox conditions associated with the O2overshoot. To better understand the mid-Paleoproterozoic ocean chemistry, we investigated the microstructures, major and trace element concentrations, Re-Os and Mo (δ98/95Mo) isotopes, and total organic carbon contents of Mn-ore and phyllite samples from the Nsuta Mn deposit in the Birimian Supergroup of Ghana which were deposited during the O2overshoot (at ~2.2 Ga). The Mn-ore samples contain early diagenetic rhodochrosite (Mn carbonate). The trace element compositions and Re-Os isotopes of the Mn-ore samples suggest that the rhodochrosite originated from primary manganese oxides (MnO2) deposited at ~2.2-Ga. The δ98/95Mo values of the least-altered Mn-ore samples range between −1.10‰ and −0.55‰ (relative to NIST3134), suggesting seawater δ98/95Mo values of 1.85 ± 0.18‰ (1SD) during the O2overshoot. Such high seawater δ98/95Mo values can be best explained by enhanced removal of isotopically light Mo through adsorption onto Mn oxides. To form extensive Mn-oxide deposits, bottom seawater with O2concentrations of > 10 μM would have expanded at ~2.2 Ga. The oxidizing conditions might have supported the emergence of stem group eukaryotes during the mid-Paleoproterozoic.