Records of past methane discharges from Holocene cold seeps of Hidaka Trough based on carbon isotope values of benthic foraminifera

Hidaka Trough, a marine hydrate reservoir in the northwestern Pacific, is characterized by its gas chimney structures, faults, methane plumes, and degassing structures, providing pathways for the upward migration of free hydrocarbon gases from deep sediments to the seabed. A comprehensive understanding of methane emissions from Hidaka Trough sediments in the past and present holds the key to predicting future scenarios of methane emission. This study applied a multi-proxy approach to three sediment cores (PC1609, PC1611, and PC1727) from cold seep regions of the Hidaka Trough in the northwestern Pacific to reconstruct the local history of methane emissions. The chronostratigraphic framework of the cores was established using accelerator mass spectrometry (AMS) radiocarbon dates and δ¹⁸O profiles of benthic foraminifera, indicating that the cores were deposited during the Holocene. The geochemical composition of hydrocarbon gases within the sediments suggested a microbial origin or a mixture of microbial and thermogenic gases.

Furthermore, porewater geochemistry revealed the presence of a shallow sulfate methane transition zone (SMTZ) very close to the seafloor, between 120 and 170 centimeters below the seafloor (cmbsf) across all three cores. The δ¹³C values of benthic foraminifera provided a clear record of methane seepage in the study area. The δ¹³C records measured on the benthic foraminifera displayed one interval with a relatively low value (−3.20 ‰) in core PC1609 (referred to as Event I) and four distinct intervals with extremely negative values (as low as −32.02 ‰) in core PC1611 (designed as Events I to IV). In contrast, in core PC1727, the values were within the normal marine range. Event I in core PC1609 coincided with the depth of the present-day SMTZ. In core PC1611, negative δ¹³C values, separated by at least one sample with common marine values, occurred at distinct intervals in both the present-day SMTZ and paleo-SMTZs, suggesting shifts in the SMTZ and changes in methane release flux in the past. The events in core PC1611 were marked by horizons of sediments containing high Ca/Ti, Ba/Ti, Mg/Ti, and Sr/Ti ratio characteristics of authigenic carbonate precipitation. Besides, during these events, visual and geochemical investigations of benthic foraminifera species exhibited the presence of authigenic calcites and framboidal pyrites on the exterior and interior of the foraminifera test walls. These authigenic calcites were enriched in Mg/Ca, Ba/Ca, and U/Ca and had relatively mineralogical characteristics identical to sediments in the same intervals, suggesting that foraminifera served as suitable nucleation sites for precipitation of authigenic carbonate and as promising indicators of authigenic contamination. We propose that in cores PC1609 and PC1727, the SMTZ has been established recently, with no evidence of past methane emissions. Conversely, in core PC1611, the SMTZ migrated several times due to changes in methane flux intensity, leading to its current position.