Hypolimnetic oxygen conditions influence varve preservation and δ13C of sediment organic matter in Lake Tiefer See, NE Germany
Hypolimnetic oxygen conditions influence varve preservation and δ13C of sediment organic matter in Lake Tiefer See, NE Germany
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DOI：
10.1007/s10933-019-00084-2
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发表时间：
2019-08-01
影响因子：
2.1
通讯作者：
Brauer, Achim
中科院分区：
文献类型：
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作者：
Draeger, Nadine;Plessen, Birgit;Brauer, Achim
沉积物有机质稳定碳同位素(Delta C-13(OM))在古环境研究中有着广泛的应用。然而，对三角洲C-13(OM)的解释仍然具有挑战性，影响三角洲C-13(OM)的因素可能并不适用于所有湖泊。C-13三角洲(OM)地层变化的常见解释包括湖泊生产力的变化或异地OM输入的变化。研究了不同氧气条件(有氧和缺氧)对铁夫尔湖沉积物中C-13(OM)值的影响。我们分析了(1)来自湖最深处的一个长沉积岩心，(2)来自浅水深度的两个短的沉积物-水界面岩心，(3)水柱中的OM，即来自沉积物圈闭。整个水柱的新鲜有机质显示出相对恒定的增量C-13(OM)值，约为-30.5ppm。在长和短的沉积物-水界面岩心中，变化良好的沉积物也得到了类似的值，约为-31ppm。相比之下，所有岩芯中未变化的沉积物的Delta C-13(OM)值明显较小(-29ppm)。不同水深的沉积物-水界面岩心的Delta C-13(OM)值对于同一时代的沉积物来说是不同的，如果这些地点沉积时的氧气条件不同，如河流保存状态所示。从公元1924-1980年在62m水深沉积的沉积物具有多样性，其增量C-13(OM)值约为-31‰，而35m水深相同年龄的沉积物没有变化，且Delta C-13(OM)值负值较小，约为-29‰。C-13增量(OM)值与C-13增量之间的关系表明，C-13增量(OM)与氧气条件有关，因为C-13增量的保存有赖于低磁缺氧。可能影响增量C-13(OM)的一个机制是在有氧条件下OM的选择性降解，使得具有较负的增量C-13(OM)的有机成分优选地被分解，从而导致剩余的未降解的OM池中的增量C-13(OM)值较小的负值。与变化良好的沉积物(类似于15%)相比，这些沉积物中较低的TOC浓度(类似于5%)支持了这些沉积物中较大程度的OM分解。即使在生产力和陆地有机质输入随时间变化很小的湖泊中，次磁氧浓度的巨大时空差异也可能是控制沉积物C-13(OM)的重要因素。
Stable carbon isotopes of sediment organic matter (delta C-13(OM)) are widely applied in paleoenvironmental studies. Interpretations of delta C-13(OM), however, remain challenging and factors that influence delta C-13(OM) may not apply across all lakes. Common explanations for stratigraphic shifts in delta C-13(OM) include changes in lake productivity or changes in inputs of allochthonous OM. We investigated the influence of different oxygen conditions (oxic versus anoxic) on the delta C-13(OM) values in the sediments of Lake Tiefer See. We analysed (1) a long sediment core from the deepest part of the lake, (2) two short, sediment-water interface cores from shallower water depths, and (3) OM in the water column, i.e. from sediment traps. Fresh OM throughout the entire water column showed a relatively constant delta C-13(OM) value of approximately -30.5 parts per thousand. Similar values, about -31 parts per thousand, were obtained for well-varved sediments in both the long and short, sediment-water interface cores. In contrast, delta C-13(OM) values from non-varved sediments in all cores were significantly less negative (-29 parts per thousand). The delta C-13(OM) values in the sediment-water interface cores from different water depths differ for sediments of the same age, if oxygen conditions at the time of deposition were different at these sites, as suggested by the state of varve preservation. Sediments deposited from AD 1924 to 1980 at 62m water depth are varved and exhibit delta C-13(OM) values around -31 parts per thousand, whereas sediments of the same age in the core from 35m water depth are not varved and show less negative delta C-13(OM) values of about -29 parts per thousand. The relation between varve occurrence and delta C-13(OM) values suggests that delta C-13(OM) is associated with oxygen conditions because varve preservation depends on hypolimnetic anoxia. A mechanism that likely influences delta C-13(OM) is selective degradation of OM under oxic conditions, such that organic components with more negative delta C-13(OM) are preferably decomposed, leading to less negative delta C-13(OM) values in the remaining, undegraded OM pool. Greater decomposition of OM in non-varved sediments is supported by lower TOC concentrations in these deposits (similar to 5%) compared to well-varved sediments (similar to 15%). Even in lakes that display small variations in productivity and terrestrial OM input through time, large spatial and temporal differences in hypolimnetic oxygen concentrations may be an important factor controlling sediment delta C-13(OM).