Abstract
The ratio of total organic carbon to total nitrogen (TOC:TN) and the stable carbon isotope ratio of organic matter (δ13Corg) are widely applied for inferring the origin of organic matter (OM) in Quaternary marine sediments. A plot of TOC:TN vs. δ13Corg is useful for such studies but is strongly based on qualitative constraints. This study is based on the qualitative characterization of the source of Quaternary OM via analysis of TOC:TN and δ13Corg signatures, but also proposes a probability parameter, which combines both signatures, to infer the amount of Terrestrial OM Input (TOMI). This index provides a method for quantifying the proportion of terrestrial OM vs. marine OMin a more comprehensive manner. The TOMI index concept was applied to a study area in theJoetsuBasin, eastern margin of theJapanSea, where previous studies have characterized theOMfrom the Last Glacial Maximum (LGM) to the present. The upwards increase in TOC indicates thatOMproduction during the Holocene was higher than during the LGM. The enriched δ13Corg signature upwards and decrease in TOC:TN suggest predominantly marine phytoplankton OM during the Holocene. Throughout the LGM, low OM production with depleted δ13Corg values and high TOC:TN values in the sediments suggest a predominantly C3 terrestrial plant source for the OM. Using these data, it was possible to calculate a proxy for a sea level variation curve during that period and to investigate the influence of the proximity of the coastal line to the continental slope on the input of terrestrial material to the basin. The proposal provides information for the application of sequence stratigraphic concepts. The TOMI index could confirm that the proximity to the shoreline and shelf break has a strong influence on the input of terrestrial material during lowstand periods.
Highlights
The identification of the sources of organic matter (OM) in marine sediments is important for inferring the contribution of terrestrial material since both terrestrial vegetation and soils can be delivered to the deeper parts of a basin along the geological time
The upwards increase in total organic carbon (TOC) indicates that OM production during the Holocene was higher than during the Last Glacial Maximum (LGM)
Unit 2 (TL-1) is a 5 - 20 cm dark gray thinly laminated mud layer, which is common in the Japan Sea
Summary
The identification of the sources of organic matter (OM) in marine sediments is important for inferring the contribution of terrestrial material since both terrestrial vegetation and soils can be delivered to the deeper parts of a basin along the geological time. For example, the sea level was lowered by tens of meters and the input of terrestrial material became high at distal sites. Sea level curves can be generated using the terrestrial OM input as a proxy. During such a sea level decline the occurrence of turbidite flows is common. In regions where there are no sandy sediments they represent distal parts of turbidite flows, so clayey sediments are dominant. In these cases, it is difficult to infer the provenance or location of the sediment source
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