A HOLOCENE HISTORY OF DYNAMIC WATER COLUMN REDOX CONDITIONS IN THE LANDSORT DEEP, BALTIC SEA
A HOLOCENE HISTORY OF DYNAMIC WATER COLUMN REDOX CONDITIONS IN THE LANDSORT DEEP, BALTIC SEA
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DOI：
10.2475/08.2016.01
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发表时间：
2016-10-01
影响因子：
2.9
通讯作者：
Lyons, Timothy W.
中科院分区：
文献类型：
--
作者：
Hardisty, Dalton S.;Riedinger, Natascha;Lyons, Timothy W.
现代波罗的海是世界上最大的人为强迫缺氧盆地。利用在综合大洋钻探计划(IODP)347考察期间从波罗的海最深和最减少的次盆地之一LandsorDepth收集的综合地球化学记录，我们探索了整个波罗的海全新世的自然缺氧程度和频率。岩心碳硫比(C/S)的显著下降表明，从波罗的海冰湖向现今咸水海的过渡发生在8.5kyB.P.左右。此后，整个沉积物中记录的微咸水沉积纹层表明持续缺氧或极低的氧气，而高钼(Mo)浓度(>；100ppm)和铁(Fe)地球化学表明水柱硫化物积累，或称Euxinia，在两个不同的腐泥岩单元沉积期间，这种积累持续超过季节性时间尺度。沉积Mo同位素的取值范围为+1.11至-0.50 permil，明显来自现代波罗的海海水的分馏(+2.26至-2.67 ppm)，因此表明每个腐泥岩只经历了微弱的和/或振荡的等温条件--与更受限制的盆地更稳定的等温条件形成对比。从下腐蚀面上方开始的三角洲Mo-98向明显更负的范围移动，表明Euxinia特别弱且振荡，相对于剖面的下部，锰(Mn)氧化还原循环对Mo沉积的贡献增强。这一结论得到了高达15%重量百分比的极端沉积锰富集的支持。我们对综合数据的解释表明，北海海水和含氧海水间歇性但主要地流入缺氧陆地深部，限制了水柱硫化物的浓度和停留时间，并导致了幕间氧化物沉积。考虑到最严重的还原条件和氧化还原不稳定时期之间的时间重叠，我们假设，正如今天所观察到的那样，主要的波罗的海资金流入导致短期不稳定，同时通过加强盐跃层来支持波罗的海的长期缺氧。最终，我们的结果表明，全新世期间比现代波罗的海更短的时期是自然发生的，但特有的动态盐分输入在历史上阻止了在其他经典受限盆地中观察到的相对更广泛和更稳定的缺氧，并可能继续这样做。
The modern Baltic Sea is the world's largest anthropogenically forced anoxic basin. Using integrated geochemical records collected during Integrated Ocean Drilling Program (IODP) Expedition 347 from the deepest and one of the most reducing sub-basins in the Baltic Sea, Landsort Deep, we explore the degree and frequency of natural anoxia through the Baltic Holocene. A marked decrease in carbon-to-sulfur ratios (C/S) from the cores indicate the transition from the Baltic Ice Lake to the current brackish sea, which occurred about 8.5 kyrs B.P. Following this, laminations throughout sediments recording brackish deposition suggest sustained anoxia or extreme low oxygen, while high molybdenum (Mo) concentrations of >100 ppm and iron (Fe) geochemistry suggest water column sulfide accumulation, or euxinia, that persisted beyond seasonal timescales during deposition of two distinct sapropel units. Sedimentary Mo isotope values range from +1.11 to -0.50 permil, which are distinctly fractionated from modern Baltic seawater (+2.26 to -2.67 parts per thousand) and thus indicate that each of the sapropels experienced only weak and/or oscillatory euxinia-in contrast to the more stable euxinic conditions of more restricted basins. A shift in delta Mo-98 starting above the lower sapropel to a distinctly more negative range suggests particularly weak and oscillatory euxinia, with an enhanced contribution of manganese (Mn) redox cycling to Mo deposition relative to the lower portion of the profile. This conclusion is supported by extreme sedimentary Mn enrichments of up to 15 weight percent. We interpret the combined data to indicate episodic but major Baltic inflow events of saline and oxygenated North Sea water into the anoxic Landsort Deep that limited the concentrations and residence time of water column sulfide and caused episodic oxide deposition. Considering the temporal overlap between the most reducing conditions and periods of redox instability, we hypothesize that major Baltic inflows, as is observed today, lead to short-term instability while simultaneously supporting longer-term Baltic anoxia by strengthening the halocline. Ultimately, our results indicate that periods more reducing than the modern Baltic Sea have occurred naturally over the Holocene, but the characteristic dynamic saline inputs have historically prevented the relatively more widespread and stable anoxia observed in other classic restricted basins and will likely continue to do so.