Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2021-01-12 , DOI: 10.1016/j.gca.2021.01.002 JinHua Liu , Lian Zhou , Qian Wang , LanPing Feng , Jun Shen , Achim D. Herrmann , Thomas J. Algeo
The North American Midcontinent Sea (NAMS) covered a vast area during Late Pennsylvanian glacio-eustatic highstands, providing a laboratory for analysis of the internal watermass dynamics of large cratonic seas (of which few exist in the Recent). In this study, a novel proxy, copper (Cu) isotopes, was used to gain a better understanding of NAMS watermass dynamics. We analyzed the 63-cm-thick Stark Shale (Dennis Formation, Missourian Stage, Upper Pennsylvanian) in the Iowa Riverton core (IRC) at a centimeter scale in order to reconstruct secular variations in Cu-isotopic compositions (δ65Cu) and other geochemical proxies. The gray shale facies yielded δ65Cu of +0.02 ± 0.06‰ (2σ, n = 14), similar to detrital Cu in modern marine sediments (δ65Cu = +0.08 ± 0.20‰; 2σ, n = 42). In contrast, the black shale facies, in which the proportion of authigenic Cu hosted by the organic fraction is 50–100% of total Cu content, exhibits heavier δ65Cuauth values, mostly between +0.09 and +0.43‰. In modern marine systems, one of the main processes leading to 65Cu-enriched sediment compositions is adsorption of aqueous Cu onto Fe-Mn particulates. The black shale facies of the Stark Shale exhibits four peaks in Mo/U ratios that are indicative of an active particulate shuttle because of the tendency of Mo to adsorb more strongly than U to Fe-Mn particulates. The Sr/Ba ratio, a paleosalinity proxy, shows correlated variations, with high values (indicative of more fully marine conditions) linked to low Mo/U and light δ65Cu, and low values (indicative of brackish conditions) linked to high Mo/U and heavy δ65Cu. These considerations suggest that the flux of isotopically heavy Cu to the sediment–water interface via Fe-Mn cycling was enhanced during brackish intervals, with subsequent reductive dissolution of Fe-Mn particulates allowing transfer of hydrogenous Cu to the organic fraction of the sediment. Control of δ65Cu by Fe-Mn cycling is supported by the results of a sequential extraction experiment showing that organic δ65Cu is positively correlated with Mo/U. These findings provide evidence of large fluctuations in watermass salinity and sea-level elevation within the NAMS at timescales of ∼104 yr during core shale deposition, as previously inferred but not conclusively demonstrated until now. The dynamics of particulate shuttle operation suggest that sea-level fluctuations must have had amplitudes of at least a few tens of meters. These fluctuations ended during the final stages of black shale deposition, probably owing to overall sea-level fall and permanent shallowing of the study site into the oxic, low-salinity surface layer of the NAMS.
中文翻译:
宾夕法尼亚后期北美中大陆海中颗粒物穿梭动力学的铜同位素证据,对冰川-恒星大小有影响
北美中大陆海(NAMS)在宾夕法尼亚州晚冰川期高冰期期间覆盖了广阔的区域,为分析大型克拉通海的内部水质动力学提供了一个实验室(最近很少有这种活动)。在这项研究中,一种新颖的替代物铜(Cu)同位素被用来更好地理解NAMS水质动力学。我们分析了在厘米级衣阿华里弗敦芯(IRC)的63厘米厚的斯塔克页岩(丹尼斯形成,密苏里阶段,上宾夕法尼亚),以便在铜同位素组成来重建长期变化(δ 65 Cu)的和其它地球化学代理。灰色页岩相产生δ 65的Cu 0.02±0.06‰(2σ,Ñ = 14),类似于碎屑的Cu在现代海洋沉积物(δ 65铜= +0.08±0.20‰; 2σ,n = 42)。与此相反,黑色页岩相,其中自生Cu的比例托管由有机部分是总Cu含量的50-100%,表现出较重的δ 65个铜AUTH值,大多之间0.09和0.43 +‰。在现代海洋系统中,导致65的主要过程之一富铜沉积物成分是将铜水溶液吸附到铁锰颗粒上。斯塔克页岩的黑色页岩相在Mo / U比上表现出四个峰值,这表明有活动的颗粒穿梭,因为Mo倾向于比U吸附到Fe-Mn颗粒上更强。的SR /钡比,盐度代理,节目相关的变化,具有高的值(指示更充分的海洋条件下)连接到低的Mo / U和光δ 65连接到高沫Cu和低的值(指示微咸水条件) / U和重δ 65铜 这些考虑表明,在微咸间隔期间,通过Fe-Mn循环增加了同位素重的Cu到沉积物-水界面的通量,随后Fe-Mn颗粒的还原溶解使得氢Cu转移到沉积物的有机部分。δ的控制65由铁-锰循环Cu的通过表明有机δ一个连续提取实验的结果支持65的Cu是积极与沫/ U相关。这些发现提供了在〜10 4时标上NAMS内水质盐度和海平面升高的巨大波动的证据。如先前所推论,但直到现在尚无定论,这在岩心页岩沉积过程中是最重要的。颗粒状航天飞机运行的动力学表明,海平面的波动幅度必须至少为几十米。这些波动在黑色页岩沉积的最后阶段结束,这可能是由于整体海平面下降和研究地点向NAMS的含氧,低盐度表层永久变浅所致。