Abstract:

Benthic foraminifera and their shell oxygen and carbon isotopes from Core 973-4, located on the slope of the Jiulong Methane Reefs cold seep area, and Core 973-5 on the edge of the Haiyang No.4 Site in the Taixinan Basin, were studied to understand the characteristics of fauna assemblages, their ecological factors, and the change in microhabitats with time. The ages of sediments from the two cores were determined to be 50 ka, which were assigned to the late Pleistocene Marine Oxygen Isotope Stage MIS 3 to early MIS 1. In total, 233 species of 79 genera were identified for the benthic faunas from the two cores. The dominant taxa from Core 973-4 were Uvigerina (23.3%),Bulimina (10.71%) and Cibicidoides (9.87%), while from Core 973-5 was Bulimina (20.6%). Generally, these two faunas were dominated by infauna taxa. Foraminiferal diversity was influenced by TOC and sediment grain sizes. A weak correlation between dominant (or common) taxa with TOC and varying degree of correlation with δ¹⁸O_{Uvigerina spp.} indicated that foraminifera taxa were also affected by other special nutrition and fluids in the cold seep areas. Over the last 50 ka, the microhabitats of benthic foraminiferal assemblages from Cores 973-4 and 973-5 changed from bivalve shell debris-authigenic carbonates (MIS 3 to early MIS 2) to bivalve-bacterial mats (late MIS 2 to early MIS 1). Correspondingly, from MIS 3 to MIS 1, the dominant taxa of foraminiferal assemblages from Core 973-4 changed from U.peregrina to Cibicidoides-Bulimina then to U.vadescens-Cibicides and from Core 973-5 changed from Chilostomella+Globobulimina to Cibicidoides then to Bulimina. Oxygen and carbon isotopic profiles of foraminiferal shells also changed with time, showing enriched δ¹⁸O (3.5‰-4.49‰) and depleted δ¹³C(-2.0‰ to -0.2‰), as results of exchanging ambient pore fluids with authigenic carbonates during the period of MIS 3 to early MIS 2 and biogeochemical processes in the microhabitats of bivalve and bacterial mats during the time of late MIS 2 to early MIS 1. Generally, the activity of methane seeping decreased gradually during the past 50 ka in these two cold seep regions. Several enhanced seeping events during this period were indicated by abnormal δ¹⁸O and δ ¹³C values. One persistently enhanced seeping event, lasted about 10 kyr (35-25 ka), was seen in Core 973-4. In Core 973-5, three short events (at 45 ka, 35 ka, and 14-12 ka) were found, among them the 45 ka event may be the strongest one as methane flux reached near the surface of the sea floor to form gas hydrates near the seafloor, where as the 35 ka event was inferred as a regional one.

Key words: benthic foraminifera, oxygen and carbon isotopes, cold seep microhabitats, MIS 3-MIS 1, Taixinan Basin