LIPID BIOMARKERS IN LATE PLEISTOCENE-HOLOCENE LAKE SEDIMENTS FROM SOUTHERN CHIHUAHUA DESERT, MEXICO
The evolution of ecosystems in the Southern Chihuahua Desert of Mexico since the Last Glacial and through the Holocene is yet to be fully characterised, creating a gap in our understanding of the ecological response to climate change in this water scarce region. Here we explore the response of lacustrine and surrounding terrestrial habitats of this region to hydrological changes during the Late Pleistocene and Holocene. Biomarker based proxy data from the Santa Clara Basin indicates dynamic and variable input of organic matter (OM) into the basin, with sources being the surrounding vegetation, bacterial biomass and aquatic microfauna. Changes in these inputs reveal distinct stages of ecosystem development over the last 18.7 ka. Overall, high lipid concentrations from 18.7 to 14.7 cal ka BP, mainly from terrestrial OM, are attributed to relatively high runoff. High proportions of archaeol, GDGT-0, and higher values of MI and GDGT-0 to crenarchaeol ratios suggest anoxic conditions for this cold period. Moreover, this interval corresponds with no ostracod preservation. Lower lipid proportions during the warmer Bølling/Allerød correspond to lower precipitation and runoff; however, lacustrine productivity increases, as reflected by short chain n-fatty acids (n-FAs) and ostracods. During the colder Younger Dryas period, higher terrestrial OM concentrations suggest that precipitation and runoff input returned to deglacial conditions; intriguingly, high archaeol and GDGT-0 proportions suggest the persistence of anoxic conditions. The early Holocene was marked by a general trend towards warmer and dryer conditions, and a sedimentary hiatus was detected from 10.7 to 6.4 cal ka BP. The sedimentary record was restored at 6.3 cal ka BP, this is attributed to the North American Monsoon reaching its maximum extent. Later on, a decreasing BIT index suggests that aquatic productivity increased gradually from 6.2 to 4.8 cal ka BP. Moreover, this reflects less runoff input but also much lower vegetation density in the surrounding region. This period represents the environmental transition to an arid climate as the Intertropical Convergence Zone shifted southward. During the last 4.8 cal ka BP, the lipid record reflects shallow lake conditions. Moreover, increasing proportions of archaeol but lower GDGT-0/Cren suggest increasing salinity. Higher proportions of sitosterol and campesterol could be reflecting drought stress of higher plants during this period. Finally, the last 2 ka represent the period of extreme drought with aquatic microbial adaptation to hypersalinity conditions (CPI, Paq, archaeol). Overall, this work reveals the marked sensitivity of the Southern Chihuahua Desert ecosystems to the climatic shifts that occurred since the Last Glacial, and this perhaps indicates their vulnerability to future global warming.