Geochemical sedimentary evidence from core 973-2 for methane activity near the Jiulong Methane Reef in the northern South China Sea

Abstract

The Jiulong Methane Reef, located on the northern slope of the South China Sea, is characterized by several features, such as bottom-simulating reflections and authigenic carbonates, which are indicative of methane seeps that are currently occurring as well as those that occurred in the past. However, to date, the effect that these methane seeps have on the sedimentary environment is not completely clear. To provide further insights into the biogeochemical processes involved in methane seeps, a 6.73 m piston core (973-2) was retrieved from this area in 2011 to perform an in-depth analysis. The chronology of the core has been established by [Formula: see text] dating, and sedimentary events since the last glacial period have been recorded. The results indicate abnormally low abundance and diversity, as well as high infaunal percentages of benthic foraminiferal assemblages in the lower part of the core from 673 to 350 centimeters below the sea floor (cmbsf), indicating impacts from methane activities. Major elemental barium, bromine, and titanium, which serve as proxies of the paleoproductivity, organic contents, and terrestrial supply, respectively, are found to make little contribution in [Formula: see text] content. However, regardless of the factors mentioned above, the [Formula: see text] content is still higher in the lower part of core, indicating the presence of authigenic carbonates. In addition, the total organic carbon/total sulfur ratios of less than 400 cmbsf are found to be beyond the normal range, which is [Formula: see text], reflecting the effects of methane oxidation. In one of the sections, the trace metals and molybdenum profiles show enrichments that are closely related to those of the methane seeps. The results indicate that this area experienced methane seep events in the last glacial period.

Characteristics of Authigenic Minerals around the Sulfate-Methane Transition Zone in the Methane-Rich Sediments of the Northern South China Sea: Inorganic Geochemical Evidence

“…The AMS 14 C dating results (Figure 2) of planktonic foraminifera at site 973-4 were collected according to Lin [47] and Pan [42]. The planktonic foraminifera ( N. dutertrei ) collected from 12 samples throughout the site yielded an inconsecutive sedimentary sequence, especially in Unit II.…”

“…Before analyzing the source of carbonate minerals in the site 973-4, the abnormal increase in carbonate content in the depth range of Unit II was analyzed (Figure 3). Some studies [42,46,48] have shown that foraminiferal assemblages in this cold seep site display high species abundance and diversity. A total of 9111 benthic foraminifera were obtained from 63 identified samples [48].…”

“…Benthic foraminifera inhabiting water–rock interfaces or sediments can effectively record changes in dissolved inorganic carbon in the surrounding environment [42,47,59]. According to the carbon and oxygen isotopes of benthic foraminifera (Figure 2) at site 973-4 reported by Zhang [48], the variation of CaCO 3 contents with depth was compared.…”

“…Then, the weight percentage of chromium reducible sulfide (CRS) in the deposit was calculated according to the weight of precipitated silver sulfide. Detailed experimental steps can be found in the report by Pan [42].…”

“…A total of 63 sediment samples were analyzed for trace elements by Agilent 7700e ICP-MS (Sample Solution Technology Co., Ltd., Wuhan, China) in the Analytical and Testing Center of Wuhan Shangpu Analysis Technology Co., Ltd. The treatment of sediment samples and the specific testing methods for major and trace elements have been described elsewhere [42]. To compare the respective authigenic enrichments of the trace metals in the sediment, enrichment factors (EF) were used, which are defined as XEF = [(X/Al) sample / (X/Al) PAAS], where X and Al represent the weight concentrations of elements X and Al, respectively.…”