Carbon and oxygen isotope characteristics of foraminiferan from northern South China Sea sediments and their significance to late Quaternary hydrate decomposition

Abstract

Carbon and oxygen isotope and dating analyses of foraminiferan in sediment cores collected from three different areas of the northern slope of the South China Sea were conducted, in order to examine the records of the gas hydrate decomposition events since the late Quaternary under the conditions of methane seepage. The results show that: 1) the delta C-13 values of the benthic foraminiferan Uvigerina spp. (size range of 0.25-0.35 mm) are from -0.212% to -0.021% and the delta O-18 values of the planktonic foraminiferan Globigerinoides ruber (size range of 0.25-0.35 mm) are from -0.311% to -0.060%; 2) three cores (ZD2, ZD3 and ZS5) from the bottom of a hole are aged for 11 814, 26 616 and 64 090 a corresponding to the early oxygen isotope stage (MIS) I, III and IV final period, respectively; 3) a negative-skewed layer of carbon isotope corresponds to that of MIS II (cold period), whose degree of negative bias is -0.2%; and 4) the delta C-13 compositions of foraminiferans are similar to those of the Blake Ridge and the Gulf of Mexico sediments of the late Quaternary. According to the analysis, the reasons for these results are that the studied area is a typical area of methane seep environment in the area during MIS II due to the global sea-level fall and sea pressure decrease. Gas hydrate is decomposed and released, and a large number of light carbon isotopes of methane are released into the ocean, dissolved to inorganic carbon (DIC) pool and recorded in the foraminiferan shells. A pyrite layer developed in the negative bias layers of the foraminiferans confirms that the delta C-13 of foraminiferans is more affected by methane and less by the reduction of marine productivity and early diagenesis. The use of foraminiferan delta C-13 could accurately determine late Quaternary hydrate release events and provide evidence for both reconstructing the geological history of methane release events and exploring natural gas hydrate.National Natural Science Foundation of China [40976035]; National Basic Research Program of China [2009CB219501]; Special Program of Comprehensive Survey and Assessment Offshore China Sea [908-ZC-I-07

Foraminifera associated with cold seeps in marine sediments

“…The isotopic composition of seawater, including both the pore water in seafloor sediments and the upper water body, undergoes significant changes as a result of the reformative processes of seepage fluids and methane-related biogeochemical processes, making the observation of low d 13 C and high d 18 O anomalies recorded by foraminifera shells to be widely considered as a marker of methane release at cold seeps (Li et al, 2010;Martin et al, 2010;Panieri et al, 2012). So far, the isotopic composition analysis of foraminifera has revealed the occurrence of methane-release events from various cold seep environments, including the Gulf of California (Keigwin, 2002), the Santa Barbara Basin (Kennett et al, 2000;Hill et al, 2003;Cannariato and Stott, 2004), the Black Ridge (Bhaumik and Gupta, 2007), the Cascadia margin (Hill et al, 2004;Li et al, 2010), the Peru shelf margin (Wefer et al, 1994), the West Svalbard continental margin (Panieri et al, 2014), the Norwegian continental margin (Hill et al, 2012), the Greenland Sea (Smith et al, 2001;Millo et al, 2005), the Fram Strait (Consolaro et al, 2015), the Mediterranean Sea (Panieri et al, 2012), the Japan Sea (Ohkushi et al, 2005;Uchida et al, 2008), the South China Sea (Lei et al, 2012;Wang et al, 2013;Chang et al, 2015;Zhuang et al, 2016), and the Weddell Sea (Thomas et al, 2002) (Figure 1). Additionally, the application of the isotopic composition analysis of foraminifera has enabled the interpretation of previous methanerelease events in various geological settings, including the Northern Apennines (Italy) (Panieri et al, 2009), Western Washington State (USA) (Martin et al, 2007), and the Sunda arc (Indonesia) (Wiedicke and Weiss, 2006).…”

Differences in the Sulfate–Methane Transitional Zone in Coastal Pockmarks in Various Sedimentary Environments

“…A sharp linear reduction in pore water sulfate concentrations and a relatively shallow sulfate-methane transitional zone (SMTZ) reflect a high intensity of methane flux through the underlying sedimentary layers and suggest an anoxic sedimentary environment [8,24,25]. Thus, sulfate and halide concentrations, a high intensity of methane flux, authigenic carbonate minerals, and other geochemical indicators are significant factors in studies on early diagenesis and the sedimentary environment [26][27][28]. Estuaries and coasts are the convergence zones of the land and sea, into which large amounts of terrestrial organic matter are input every year [29].…”

Differences in the Sulfate-Methane Transitional Zone in Coastal Pockmarks in Various Sedimentary Environments

“…A sharp linear reduction in pore water sulfate concentrations and a relative shallow sulfate-methane transitional zone (SMTZ) reflect a high intensity of methane flux through the underlying sedimentary layers and suggest an anoxic sedimentary environment [8,24,25]. Thus, sulfate and halide concentrations, a high intensity of methane flux, authigenic carbonate minerals and other geochemical indicators are significant factors in studies on early diagenesis and the sedimentary environment [26][27][28]. Estuaries and coasts are the convergence zones of the land and sea, into which large amounts of terrestrial organic matter are input every year [29].…”