Alkenones analyses
Freeze-dried sediment samples were extracted by an accelerated solvent extraction system with DCM: methanol mixture (9:1, v/v). The total lipid extraction was further separated into three fractions of increasing polarity by silica gel columns eluting with hexane, DCM, and methanol subsequently. The mid-polar fractions containing alkenones were analyzed on a gas chromatography–flame ionization detection Agilent 7890N Series instrument with mid-polarity column RTX-200 (105 m × 250 μm × 0.25 μm) with the oven program described in Zheng et al. (2017).