Uranium and V isotope variations in Archean sediments: Evidence for metal mobilization and enzymatic reduction in an overall anoxic world?

在这项研究中，我们希望分析黑色页岩、碳酸盐和带状铁地层（bif）的U和V同位素组成，以探索大氧化事件（GOE）之前海洋的氧化还原演化。我们还旨在获得地球上首次出现金属还原微生物的证据。微生物通过氧化有机碳获得能量，同时还原硫、硫酸盐或氧化金属，它们可能是地球上最古老的生命形式之一。根据几项研究，它们可能存在于相对富氧的泻湖中，这些泻湖可能在GOE之前的100 Ma就已经存在了。其中许多研究使用了氧化还原敏感元素的浓度和同位素组成，这些元素在缺氧条件下是不动的，但在更富氧的条件下会在海洋和沉积物中被动员和富集，包括S、Cr、Mo和u。铀同位素可能是一个特别令人兴奋的氧化还原指标，因为，根据几项调查（包括Bernier Latmani和Weyer周围的小组的最新发现），酶促U还原似乎产生了不同于非生物U还原的明显的U同位素特征。低温氧化还原反应中V同位素分馏的机理尚不清楚；然而，我们对黑色页岩的初步研究结果表明，在V还原过程中发生了显著的同位素分馏，并且V的同位素特征可以与U的同位素特征相耦合。在本项目中，我们希望利用太古宙和早元古代黑色页岩、碳酸盐岩和带状铁地层中U和V同位素的变化来寻找氧化还原相关的氧化还原敏感金属动员的第一个证据。结合其他氧化还原示踪剂的先前发现，我们想要表征金属动员和随后的还原和沉积过程中的条件。特别是，我们将寻找证据，如果金属还原是酶驱动的。我们将重点关注太古宙-元古宙过渡和GOE的样品，包括黑色页岩，其中其他金属同位素氧化还原示踪剂（例如Mo， Cr）表明这些金属的动员和随后的还原，之前已经研究（或将在平行项目中）的碳酸盐的C和S同位素，以及太古宙的bfis，之前已经用其他氧化还原代用物进行了表征。此外，我们还将研究更古老的样品，例如来自Dresser组（西澳大利亚）的3.5 Ga旧沉积物，其S同位素表明微生物S减少。总体目标将是检查U和V同位素与其他氧化还原敏感元素的同位素特征的耦合，以获得有关(1)氧化还原相关的金属动员和(2)生物与非生物还原的贡献的信息。

In the proposed study we want to analyse U and V isotope compositions of black shales, carbonates and banded iron formations (BIFs) to explore the redox evolution of the oceans prior to the great oxidation event (GOE). We also aim to obtain evidence for the first occurrence of metal reducing microorganisms on Earth. Microorganisms that gain energy by the oxidation of organic carbon, while reducing sulfur, sulfate or oxidized metal, may be among the oldest life forms on Earth. These may have existed in relatively oxygen-rich lagoons that may have existed already a few 100 Ma before the GOE as indicated by several studies. Many of these studies used concentrations and isotope compositions of redox sensitive elements that are immobile at anoxic conditions, but are mobilized and enriched in the oceans and sediments under more oxic conditions, including e.g. S, Cr, Mo and also U. Uranium isotopes may be a particular exciting redox indicator, as, according to several investigations (including recent findings of the groups around Bernier Latmani and Weyer), enzymatic U reduction appears to result in a distinct U isotope signature that is different to that of abiotic U reduction. Little is yet known about V isotope fractionation during low-temperature redox reactions; however, our preliminary results on black shales indicate that significant isotope fractionation occurs during V reduction and that isotopic signatures of V can be coupled with those of U.In the proposed project, we want to use the variations of U and V isotope in Archean and early Proterozoic black shales, carbonates and banded iron formations to search for the first evident of redox-related mobilization of redox sensitive metals. In combination with previous findings from other redox tracers, we want to characterize the conditions during metal mobilization and subsequent reduction and deposition. In particular, we will search for evidence, if metal-reduction was enzymatically-driven. We will focus on samples across the Archean-Proterozois transition and the GOE, including black shales, for which other metal isotopic redox tracers (e.g. Mo, Cr) indicate mobilization and subsequent reduction of these metals, carbonates which have previously been investigated (or will be in a parallel project) for C and S isotopes, as well as Archean BIFs which have previously been characterized by other redox proxies. Furthermore, we will also investigate older samples, e.g. the 3.5 Ga old sediments from the Dresser formation (Western Australia), for which S isotopes indicate microbial S reduction. The overall goal will be to examine the coupling of U and V isotopes with the isotopic signature of other redox-sensitive elements to gain information on (1) redox-related metal mobilization and (2) the contribution of biotic versus abiotic reduction.