Late Holocene mangrove development and response to sea level change in the northwestern South China Sea

Mangroves,widely distributed along the coasts of tropical China,are influenced by Asia monsoon,relative sea level change and enhanced human activity.To predict the impacts of future climate change on mangrove ecosystems,it can be understood by reconstructing past mangrove dynamics using proxies preserved in coastal sediments.In this study,we quantitatively partitioned buried organic matter(OM)sources,collected from a vulnerable mangrove swamp in the Qinzhou Bay of northwestern South China Sea,using a ternary end-member mixing model of δ^13C and C:N values.Mangrove-derived OM(MOM)contribution was used as a tracer for mangrove development since 2.34 cal ka BP.This information,together with paleoclimate records(i.e.,speleothem δ^18O values,sea level change,grain size parameters)and human activity,was used to divide mangrove development into three stages during the late Holocene:relative flourish(2.34-1.13 cal ka BP),relative degradation(1.13-0.15 cal ka BP)and further degradation(0.15-0 cal ka BP).Before 1.13 cal ka BP,mangroves flourished with a high MOM contribution((88.9±10.6)%),corresponding to stable and high sea level under a warm and humid climate.After 1.13 cal ka BP,rapid fall in relative sea level coupled with the strengthening of the Asian winter monsoon,resulted in mangrove degradation and MOM reduction((62.4±18.9)%).Compared with air temperature and precipitation,the relative sea level fall was the main controlling factor in mangrove development before entering the Anthropocene(the time of the Industrial Revolution).After^150 cal a BP,reclamation of mangrove swamps to shrimp ponds is the main factor causing mangrove degradation and MOM reduction.

Evolution of the North Atlantic Current and Barents Ice Sheet as revealed by grain size populations in the northern Norwegian Sea during the last 60 ka

The grain size distribution of bulk sediment samples was decomposed in a core to reconstruct paleoceanographic evolution over the past 60 ka in the northern Norwegian Sea.The results show that sediments consisted of 3–4 grain populations derived from the North Atlantic Current(NAC)and Barents Ice Sheet(BIS).The grain size data suggest three palaeoceanographic evolution stages:(1)an environment affected by BIS and NAC and changed with the interstadial/stadial transition in phase with the Greenland ice-core record at 60–31 ka BP,during which discharge of icebergs and the content of the coarsest population containing ice-rafted debris(IRD)in the sediments increased significantly during stadial,while the fine silt population containing volcanic glasses increased with the enhancement of NAC during the interstadial;(2)an extreme environment controlled by BIS at31–13 ka BP.BIS reached to its maximum at about 31 ka BP and the turbid plumes that formed at the leading edge of BIS contributed to a significant increase in the clayey population in sediments.Icebergs drained into the northern Norwegian Sea with periodical calving of the BIS at 31–19 ka BP.Subsequently,the ablation of the BIS discharged massive floods with clayey sediments and icebergs into the Norwegian Sea at 19–13 ka BP,resulting in a constant increase in clay and IRD in sediments;and(3)a marine environment similar to the present one under the strong influence of NAC following the complete melting of the BIS after 13 ka BP,NAC is the dominant transport agent and no IRD occurred in sediments.The fine silt populations containing volcanic glasses transported by NAC significantly increased.