Geochemical Proxies Preserved in Fossils and Lake Sediments

This dissertation research has used geochemical tools to reconstruct paleoecology and paleoenvironment with the goal to better understand the effects of long-term climate change on terrestrial ecosystems and mammalian evolution in South and Southwest China and on storm activity in South Florida. The major results are summarized below: The role of climate change in the evolution and diversification of hominoids remains a hotly debated issue. Stable isotope analyses of fossil mammals that coexisted with the hominoids can provide insights into hominoid paleoenvironments and shed light on this debate. In this research, stable carbon and oxygen isotope analyses were performed on tooth enamel samples from a variety of Pleistocene mammals including pandas, deer, elephants, pigs, rhinos, and bovids from two hominoid fossil localities (Yugong Cave and Baxian Cave) in South China. Enamel-δ¹³C values indicate that most of the mammals living in the study area during the late Middle Pleistocene had C₃-based diets but a small number of individuals consumed some C₄ grasses. This indicates the presence of C₄ plants in the region during the late Middle Pleistocene, most likely in patches of open areas in a predominantly forested environment. However, during the early Late Pleistocene, all the mammals examined had C₃-based diets, except one bovid and one panda that may have ingested small amounts of C₄ plants. This indicates a dense forested environment with little C₄ grasses during the early Late Pleistocene. Like the Early Pleistocene pygmy panda (Ailuropoda microta) from Yanliang Cave, the late Middle Pleistocene Ailuropoda baconi from Yugong Cave and the early Late Pleistocene Ailuropoda melanoleuca from Baxian Cave had higher mean diet-δ¹³C values than other co-occurring herbivores, indicating they preferred relatively open forest habitats and had more restricted diets compared to other mammals. The reconstructed mean paleo-meteoric water δ¹⁸Ow values are lower than the annual average δ¹⁸Ow value of modern precipitation in the region, suggesting that the climatic conditions during the times when these Pleistocene mammals were alive were colder and/or wetter than today. In addition, δ¹⁸O values of the obligate drinkers (pigs, rhinos, bovids) display an overall decreasing trend, accompanied by increased range of δ¹⁸O variations, from the Early Pleistocene to the early Late Pleistocene. This suggests that the regional climate became colder and/or wetter, with increased seasonality, from the Early Pleistocene to the early Late Pleistocene, likely related to intensified glaciation. The change in climate to colder conditions may be responsible for the extinction of the Gigantopithecus in this region. The Late Miocene witnessed significant changes in climate globally and was an important time in the evolution of hominoids. In this research, the isotopic compositions of Late Miocene mammalian teeth and freshwater shells from Shuitangba (Yunnan Province, China) – an important refugium for hominoids, were analyzed and used to reconstruct paleoenvironmental conditions in the area. δ¹³C values of fossil enamel samples range from –15.5‰ to –1.0‰, with a mean of –11.3 ± 2.3‰, and δ¹⁸O values vary from –14.2 to –4.6‰, averaging –9.7 ± 2.2‰. Reconstructed diet-δ¹³C values for these fossil mammals indicate that although they fed mostly on C₃ plants, many of them consumed some C₄ vegetation. This suggests that the local ecosystem contained C₄ grasses but was dominated by C₃ plants. The intra-tooth δ¹⁸O variation of fossil herbivores is larger than what is found in modern herbivores. Reconstructed paleo-meteoric water δ¹⁸Ow values are on average lower than those inferred from modern samples and also lower than the average δ¹⁸Ow values of modern precipitation in the region. Similarly, δ¹⁸O values of fossil freshwater shells display a larger seasonal variation and are significantly lower than modern shells from Fuxian Lake in the same region. Thus, the δ¹⁸O data from both mammalian teeth and freshwater mollusk shells support a wetter climate, possibly with a stronger precipitation seasonality in the Late Miocene than today. Comparison of clumped isotope temperatures from fossil and modern shells suggests a mean annual temperature of ~15 to 16°C in the Shuitangba area in the Late Miocene, which is ~3 to 4°C higher than that of today. Taken together, our isotope results suggest that C₄ grasses existed in local ecosystems in the area, likely in patches of grasslands or wooded grasslands in a mostly forested environment in the Late Miocene when the local and regional climate was warmer and wetter than today. Comparison of the δ¹³C records from Yunnan and the Siwalik region suggests that C₄ biomass spread earlier in the Indian subcontinent on the southwest side of the Tibetan Plateau than in Yunnan on the southeast side of the Plateau. The transition from a relatively wet habitat to a more open and drier habitat is also more pronounced and begins earlier in the Siwalik region, relative to Yunnan. These regional differences in climate and ecosystem evolution may be linked to the unique growth history of the Himalayan-Tibetan Plateau. Coastal lake sediments are important archives to understand long-term variability of paleoclimate. A nearly 1700-year-long paleostorm record was reconstructed based on multiple organic geochemical proxies preserved in a sediment core from a coastal lake in South Florida. The OPGs data show that significant variations in δ¹³C, δ15N, C%, N% and C/N with depth, reflecting changes in the lake environment that affected the processes delivering water/sediment to the lake as well as biological productivity with the lake. Analysis of modern organic materials in the lakes and their surrounding areas shows that the major sources of sedimentary organic matter in the lake are aquatic and terrestrial C₃ vegetation. Based on our proposed criteria, a total of 33 events were identified in our sediment core, including 10 seawater (SW) flooding events and 23 freshwater (FW) flooding events. The proxy storm record indicate shows two active periods of intense storm activities from 1540 to 1200 cal yr BP and from –30 to –68 cal yr BP (or 1980 to 2018 C.E.). An inactive period was also identified between 365 and –5 cal yr BP. One of the active periods (1540 to 1200 cal yr BP) coincides with a period of heightened hurricane activities in the grain-size-based paleorecord from Salt Pond in Massachusetts. This active period also correlates with elevated sea surface temperatures (SSTs) in both the Gulf of Mexico (GoM). The calm patterns observed in our record (365 to –5 cal yr BP) coincides with an ENSO-Ecuador record which indicates a distinct pattern of less intense ENSO events over the past 500 years. Our research provides new insights into the understanding of paleostorm activities in the sand-limited South Florida region.