Outsized Turbidity Currents as a Primary Mechanism for Neoproterozoic Organic Carbon Delivery to the Deep Sea

Celeste M. Cunningham, R. Arnott
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Abstract

Levees in modern deep-marine systems have been shown to sequester significant amounts of organic carbon due largely to their expanse and high rates of sedimentation. However, relatively few studies have examined organic carbon sequestration in ancient deep-marine leveed slope channel systems. Physical and geochemical analyses of well-exposed levee deposits in the Neoproterozoic Windermere Supergroup in B.C., Canada have shown that intervals of organic-rich (up to 4% TOC) strata correlate with conditions of elevated sea level and primary productivity on the shelf. Organic matter (OM) occurs primarily as micro- to nano-scale carbon adsorbed onto the surface of clay grains and notably occurs mostly in anomalously thick, mud-rich sandstone beds that are interspersed within successions of thin-bedded, comparatively organic-poor turbidites. The concentration of organic carbon in thick beds suggests that even when primary productivity is high it only becomes mobilized in significant quantities into the deep sea by uncommon, outsized turbidity currents. Although markedly more common in organic-rich intervals, thick, organic-rich beds occur also in organic-poor levee deposits, suggesting that the occurrence and frequency of outsized flows may be linked to primary productivity on the shelf. High rates of OM production and fallout would result in rapid accumulation of OM on the seafloor that then binds and provides mechanical strength to the accumulating sediment. Later this overthickened, organic-rich sediment pile becomes gravitationally unstable and ultimately remobilized downslope. These failure events create large, surge-like flows that are considerably thicker than the depth of the slope channels through which they travel. Accordingly, continuous overspill over the channel margins results in the deposition of an anomalously thick, sand- and organic-rich bed. These episodic events not only deplete the outer continental shelf of OM, but apparently also reduces the gradient slope of the local seabed, which then results in the more typical smaller, channel-confined organic-poor turbidity currents. Additionally, the abrupt and single-bed occurrence of OM-rich strata suggests that the buildup of organic-rich strata and seafloor stabilization was rapid but only of limited duration. Significantly, this study suggests that outsized turbidity currents that originate on the outer continental shelf are the primary mechanism for organic matter delivery to the deep sea, at least in pre-vegetation times, and that flow size and frequency, in addition to primary productivity, exerts an important control on the distribution of organic carbon in deep-sea sediments.
超大浊度流是新元古代深海有机碳输送的主要机制
现代深海系统中的大堤已被证明能够隔离大量的有机碳,这主要是由于它们的广阔和高沉降率。然而,对古代深海高架斜坡河道系统有机碳固存的研究相对较少。对加拿大bc省新元古代温德米尔超群中暴露良好的堤岸沉积物的物理和地球化学分析表明,富有机质(TOC高达4%)地层的间隔与海平面上升和陆架初级生产力的条件有关。有机质(OM)主要以吸附在粘土颗粒表面的微到纳米级碳的形式存在,并且主要存在于异常厚、富含泥质的砂岩层中,这些砂岩层穿插在薄层状、相对缺乏有机物的浊积岩序列中。厚厚的地层中有机碳的浓度表明,即使初级生产力很高,它也只有在罕见的超大浑浊流的作用下才会被大量动员到深海中。尽管在富有机质层段中明显更为常见,但厚的富有机质层也出现在缺乏有机质的堤坝沉积物中,这表明特大流的发生和频率可能与陆架上的初级生产力有关。高速率的OM产生和沉降将导致OM在海底的快速积累,然后结合并为积累的沉积物提供机械强度。后来,这个过厚的、富含有机物的沉积物堆变得不稳定,最终在下坡重新移动。这些破坏事件产生了巨大的、浪涌状的水流,其厚度远远大于它们所经过的斜坡通道的深度。因此,河道边缘的连续溢水导致异常厚的富砂富有机质层的沉积。这些偶发性的事件不仅耗尽了外大陆架的OM,而且显然也减少了当地海床的坡度,这就导致了更典型的更小的、受渠道限制的缺乏有机的浑浊流。此外,富有机质地层的突然和单层出现表明富有机质地层的形成和海底稳定是快速的,但持续时间有限。值得注意的是,这项研究表明,至少在前植被时代,源自外大陆架的超大浊度流是有机物向深海输送的主要机制,而且除了初级生产力之外,流的大小和频率对深海沉积物中有机碳的分布具有重要的控制作用。
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