Indicative Significance Of 3-hydroxy Fatty Acids And Branched GDGTs From The Yellow Sea And East China Sea To Seawater Temperature And PH Over The Past 60 Years

Reconstructions of paleo-climate and environment are important for predicting their future variation trends and developing coping strategies.Lipids in microbial membranes are useful tools for paleo-climate and environment reconstruction because they are sensitive to environment changes and are stable in geological carriers.Much attention has been paid to the climate and environment changes in the Yellow Sea and East China Sea since they are typical and vital marginal seas in the western Pacific Ocean.Exploring the climate and environment changes in the Yellow Sea and East China Sea is helpful for understanding sea-land interaction,global climate change and the relationship between human activities and marine ecological systems.The marine sediments are one of the important archives which record the environmental changes and major climate events.The Yellow Sea and East China Sea are thus ideal locations to reconstruct paleo-climate and environment given the widely distributed mud areas.The distributions and sources of 3-hydroxy fatty acids(3-OH-FAs)and branched GDGTs(brGDGTs)as well as the relationship between their distributions and seawater environmental conditions were explored by analyzing the 3-OH-FAs and brGDGTs in particulate matters,surface sediments and sediment core from the Yellow Sea and East China Sea.The main findings and insights are as follows:(1)Based on the distributions and sources of 3-OH-FAs and brGDGTs,as well as their relationships with seawater temperature and pH,the proxy RNT14(ratio of normal to total C14 3-OH-FAs)was proved to have potential for indicating seawater temperatures.The 3-OH-FAs in particulate matters from the coastal East China Sea are mainly produced in-situ and their distributions are influenced by seawater temperature rather than pH.BrGDGTs in particulate matters within the Changjiang River estuary are mainly derived from terrestrial inputs and record mean annual air temperature and soil pH in low Changjiang River.BrGDGTs in particulate matters in adjacent East China Sea have mixing sources of in-situ production and terrestrial inputs and are unable to accurately reflect terrestrial or marine environmental changes.The bottom particulate matter may be an important source of sedimentary 3-OH-FAs,whereas the contribution of particulate matters at different depth to sedimentary brGDGTs remains unclear.Besides,the 3-OH-FA-based temperature and pH proxies in surface sediments are different from that in particulate matters due to their different time scales.The 3-OH-FAs in particulate matters have more anteiso and less long chain components than that in soils,suggesting that the former are mainly produced in-situ.The distribution of 3-OH-FAs in particulate matters is controlled by in-situ seawater temperature that the relative abundance of normal homologues which have high phase transition temperatures are reduced at low temperatures.The 3-OH-FA-based RNT14(n C14/C14)proxy has a strong significant correlation with in-situ seawater temperature.In contrast,the in-situ seawater pH is not a main factor controlling the distribution of3-OH-FAs in particulate matters.The seawater pH is relatively stable and the 3-OH-FA producers may be not that sensitive to pH changes in such a limited pH range.The ∑Ⅲa/Ⅱa and #Ringstetra proxies for brGDGTs in particulate matters within the Changjiang River estuary are low and the composition of particulate brGDGTs are similar to terrestrial brGDGTs,suggesting that terrestrial inputs make sense to these particulate matters.The temperature estimated by brGDGT-based MBT/CBT proxies within the Changjiang River estuary are consistent with the MAAT and soil pH in the low Changjiang River.The contribution of in-situ brGDGT production is relatively high outside the Changjiang River,however,the terrestrial inputs could not be ignored.The particulate brGDGTs could not accurately reflect terrestrial or marine environmental changes,which may be due to the mixing signals of terrestrial and marine brGDGTs or the limited temperature and pH range.The sedimentary 3-OH-FAs are likely to be derived from bottom particulate matters because of their similar distributions.The sedimentary and particulate brGDGT distribution is similar to each other,however,the contribution of particulate at different depth to sedimentary brGDGTs remains unclear.The 3-OH-FA-based temperature and pH proxies in surface sediments are different from that in particulate matters due to their different time scales with the former accumulating temperature and pH signal over longer time scales.(2)The proxy RAI15 was proposed to estimate bottom seawater temperature and in warm seasons based on the relationships between sedimentary lipid biomarkers and environmental parameters of different depths and seasons.The3-OH-FAs in surface sediments from Yellow Sea and East China Sea are mainly derived from in-situ production at bottom layers during warm seasons and thus record the growth temperature and pH.The brGDGTs in surface sediments are mainly derived from in-situ production at surface layers without seasonal bias and record annual mean surface temperature.However,the sedimentary brGDGTs are not sensitive to seawater pH changes.The sedimentary 3-OH-FAs with higher abundance of anteiso homologues and lower abundance of long chain homologues than that in soils are mainly derived from in-situ production.The correlations of 3-OH-FA-based temperature proxies with annual mean bottom seawater temperature are stronger than with surface seawater temperature.The correlations of the proxies with bottom seawater temperature in warm seasons are stronger than in cold seasons.This suggests that the marine 3-OH-FAs are mainly produced in bottom water during warm seasons.The novel proxy RAI15(a C15/i C15)exhibits a stronger correlation with bottom seawater temperature in warm seasons than other proxies.Moreover,the 3-OH-FA-based pH proxies are significantly correlated with seawater pH of warm seasons.The local calibrations of RAI15 with bottom seawater temperature and RIN with bottom seawater pH in warm seasons are thus established.The sedimentary brGDGTs with more methyl and cyclopentane groups than that in soils are mainly derived from in-situ production.The relationship of seawater temperature with brGDGT distribution is better captured by MBT’6Me than MBT’5Me.The correlations of brGDGT-based temperature proxies with annual mean surface temperature are stronger than with bottom seawater temperature and have no seasonal bias,suggesting that the marine brGDGTs are mainly produced in surface water during the whole year.The relationship between fractional abundance of brGDGTs and annual mean surface seawater temperature is established by best subset regression.The brGDGT-based pH proxies are significantly correlated with annual mean surface seawater pH,however,the relationships are conflict to microbial membrane adaption mechanisms.The distributions of sedimentary brGDGTs are mainly controlled by seawater temperature rather than pH due to the limited seawater pH range.(3)The bottom seawater temperature in warm seasons at a typical station(122.45°E,30.90°N)in the East China Sea was reconstructed using proxy RAI15 of3-OH-FAs from a sediment core.The RAI15-estimated temperatures were in accordance with the instrumental data and both showed an increasing tendency from 1966 to 2019,suggesting that RAI15 is reliable for reconstructing bottom seawater temperature in warm seasons.The bottom seawater pH in warm seasons was reconstructed using proxy RIN.The RIN-estimated pH increased from 1966 to 2019,with a sharp decrease from 2003 to 2012.The increasing seawater pH may be a result of the resuspension of sediments because the dissolution of marine silicate and carbonate minerals could counteract the coastal acidification.The brGDGTs in sediment core are largely influenced by terrestrial inputs and IRIIa’is a potential proxy for soil pH in low Changjiang River.The 3-OH-FAs in sediment core are unlikely to be influenced by terrestrial inputs and mainly derived from in-situ production.The RAN15-estimated temperature is close to actual bottom water temperature in warm seasons and similar variation tends are found between them.The RIN-estimated pH is slightly higher than actual seawater pH and exhibits an increasing trend to some extent from 1966 to 2019.This is likely because that the dissolution of marine silicate and carbonate minerals contributes to CO2 consumption in this station.The 3-OH-FAs in sediment core are largely influenced by terrestrial inputs and have potential to reflect terrestrial environments.The MBT/CBT-estimated temperature is inconsistent with the actual mean annual air temperature in low Changjiang River,however,similar values and variation trend between brGDGT-estimated and actual temperatures are observed during certain period.The IRIIa’-estimated pH is in accordance with the soil pH in low Changjiang River and exhibits a decreasing trend from 1966 to 2019,suggesting that IRIIa’ is a potential soil pH proxy for coastal sediments.