Paleoenvironment and organic matter enrichment of the Carboniferous volcanic-related source rocks in the Malang Sag, Santanghu Basin, NW China

Volcanic activity was quite frequent during the deposition of the Late Carboniferous Ha’erjiawu Formation in the Santanghu Basin. The petrology and organic and inorganic geochemical indicators were used to investigate hydrocarbon potential, paleoenvironmental conditions and organic matter enrichment of the mudstones. The results show that the oil generation capacity of the Ha’erjiawu Formation mudstones, which has abundant oil-prone organic matter (Type II kerogen with hydrogen index values mainly ranging from 250 to 550 mg HC/g TOC) in mature stage (Tmax values mainly ranging from 435 to 450 °C), is considerable. The Ha’erjiawu Formation was deposited in a dysoxic, freshwater-mildly brackish, and warm-humid environment. During its deposition, the Ha’erjiawu Formation received hydrothermal inputs. The volcanic hydrothermal activities played an important role in the organic matter enrichment. In addition, the total organic carbon (TOC) is significantly positively correlated with the felsic mineral content, but it is negatively correlated with the carbonate mineral content and C27/C29 ratios, indicating that terrigenous organic matter input also contributed to the primary productivity in the surface water. Therefore, the formation of the high-quality source rocks in the Ha’erjiawu Formation was jointly affected by the hydrothermal activity and the terrigenous organic matter input.

Has primary production declined in the Salish Sea?

Declining primary production has been proposed as an explanation for the declines in coho and chinook salmon in the Salish Sea since the 1970s. Marine sediments maintain a continuous record of conditions in the overlying water. We used stable isotopes of organic carbon and nitrogen measured in twenty-one sediment cores to determine the contributions and fluxes of marine-derived and terrigenous organic matter over time. The flux of marine-derived organic matter shows no trend for at least the last 100 years. An apparent increase in the marine flux in recent years is due to remineralization of organic matter as it passes through surface sediments. In contrast, the flux of terrigenous organic matter has increased over the last century in the Strait of Georgia, while in Puget Sound, terrigenous flux peaked in the mid-twentieth century. Total primary production has neither increased nor decreased in the Salish Sea over the last century. Consequently, a decline in primary production cannot explain recent declines in fish populations.

Degradation of terrigenous organic matter on the East Siberian Arctic Shelf assessed by lipid and lignin oxidation products

<p>Warming-induced permafrost thawing is expected to intensify the remobilization of terrigenous organic matter (terrOM) to the East Siberian Arctic Shelf (ESAS) via increasing river discharge and coastal erosion. Earlier studies have focused on source apportionment and transport of terrOM, with less emphasis on its degradation state during cross-shelf transport. Since degradation of terrOM is the link between permafrost thawing and release of GHGs such as CO<sub>2</sub>, this study focuses on the degradation characteristics. Hence, the main objective of this study is to assess the patterns of terrOM degradation across the East Siberian Arctic Shelf using molecular proxies that are specific to terrOM.</p><p>Lignin phenols and high molecular weight (HMW) n-alkanes and n-alkanoic acids are only produced by terrestrial plants which make them suitable biomarkers to assess degradation of terrestrial material throughout the ESAS. The lignin-based proxies acid to aldehyde ratios of vanillyl (Vd/Vl) and syringyl (Sd/Sl) structural units, as well as the ratio of 3,5-dihydroxybenzoic acid over vanillin (3,5-Bd/V) are expected to increase during degradation under oxic conditions. Fresh terrestrial plant material is predominated by long odd-numbered (>C<sub>25</sub>) and even-numbered (>C<sub>24</sub>) carbon chain length of n-alkanes and n-alkanoic acids, respectively. This dominance is described in the Carbon Preference Index (CPI). When degradation takes place, CPI values decrease accordingly, describing how much of the original material was preserved. Ratios of HMW n-alkanoic acids to HMW n-alkanes are also expected to decrease during microbial degradation owing to preferential loss of functional groups.</p><p>The data show increasing Vd/Vl, Sd/Sl and 3,5-Bd/V ratios, and decreasing HMW n-alkanes CPI values toward the outer shelf, consistent with continuous degradation of terrOM across the ESAS. While Vd/Vl and HMW n-alkane CPI did not show strong differences between east and west, Sd/Sl ratios were highest in the outer western ESAS, and 3,5-Bd/V ratios were highest in the outer east. These differences may reflect different terrOM pools along the ESAS due to differences in vegetation zones releasing the input material through river discharge and coastal erosion. In contrast, HMW n-alkanoic acid to HMW n-alkane ratio and HMW n-alkanoic acid CPI showed inconsistent patterns across the ESAS; reasons for it are currently being investigated. These results will also be complemented by additional biomarkers to better understand the degradation of terrOM during cross-shelf transport.</p>