The high sedimentation rate in the southern Okinawa Trough provides an excellent sedimentary archive for the study of continent-ocean interactions in the marginal sea of East Asia and the reconstruction of chemical weathering and physical erosion history in tectonically active mountainous rivers (i.e., Taiwan rivers) in response to paleoenvironmental changes. The sediment core S3 retrieved from the southern Okinawa Trough is taken as the main research object of this dissertation. The radiocarbon age of planktonic foraminifera, the grain size, major element, trace element, Sr-Nd isotope, mass accumulation rate and heavy mineral compositions of the sediments, and the major element, trace element and morphological compositions of detrital apatite were determined to discuss the source and source rock types of terrigenous sediments in the southern Okinawa Trough since the late Holocene, reconstruct the history of fluvial discharge and silicate weathering in the source area since the late Holocene, and reveal the potential impacts of climate change, episodic events (e.g., typhoons and earthquakes) and human activities on regional physical erosion and chemical weathering intensities.
The sediment source analysis of Sr-Nd isotope data suggests that the terrigenous sediments in the southern Okinawa Trough since ca. 3000 cal yr BP were mainly sourced from the rivers in northeastern and western Taiwan. Large rivers in mainland China (e.g., the Yangtze River and the Yellow River) and rivers from eastern Taiwan, volcanic materials, and aeolian dust have not contributed significantly to the deposition of the southern Okinawa Trough over the past approximately 3000 years.
The results for provenance discrimination on the basis of detrital apatite geochemical data indicate that detrital apatite in the southern Okinawa Trough were derived from a mixture source with a wide variety of rock types. The apatite grains in Layer 1 with older sedimentation age were mainly originated from mafic/intermediate rocks, whereas the apatite grains in the three younger layers were derived from mafic/intermediate rocks and highly evolved rocks. In addition, a small part of grains in each layer might have contributed from alkaline rocks and metamorphic rocks. The principal component analysis (PCA) results of detrital apatite and regional stratigraphic evidence suggest that the terrigenous sediments in the southern Okinawa Trough are the products of weathering and erosion of sedimentary rocks and metasedimentary rocks in the river basins of northeastern and western Taiwan, and their ultimate sources include various rock types. The fluvial sediments from eastern Taiwan did not significantly contribute to the core site. There are some differences in geochemical compositions between the apatites in Layers 2-4 and apatites in Layer 1, suggesting that the weathered/eroded sedimentary rock types in the weathering profile of Taiwan river basins might have changed since Layer 2 (2010–1940 cal yr BP).
The chemical index of alteration (CIA), K/Al, αAlK, Ti/Na, and K/Na were established as the proxies of chemical weathering intensity according to the major element compositions of the sediment. In addition, the average etch pit index (EPI) was calculated to infer the variations in the regional chemical weathering intensity on the basis of the morphological characteristics of detrital apatite. The results suggest that the bedrocks in Taiwan river basins have undergone relatively constant and moderate strength of chemical alteration since ca. 3000 cal yr BP. According to the mass accumulate rate (MAR) of the sediment, MARsiliciclastic and MARTi were determined to reflect changes in the strength of fluvial discharge. The average aspect ratio (AR) and the percent of crystal surface (PCF) of detrital apatite were also constructed as fluvial discharge intensity proxies. The results indicate relatively weak terrigenous sediment supply during the interval of 3000-2100 cal yr BP, followed by a progressively increased fluvial discharge from 2100 cal yr BP to present.
By comparing the sedimentary records in Core S3 with previously reported climate and human activity records in Taiwan, indicating that the increased terrigenous sediment input from Taiwan to the southern Okinawa Trough since ca. 3000 cal yr BP was closely related to the changes in precipitation and anthropogenic processes on Taiwan Island. The strength of fluvial discharge was relatively weak during the interval 3000-2100 cal yr BP, corresponding well with the small rainfall on Taiwan during this period; the precipitation has increased and maintained at a relatively constant level from 2100 cal yr BP to present, which was consistent with a significantly enhanced fluvial discharge; the progressively increased terrigenous sediment supply since ca. 1500 cal yr BP was likely to be caused by the joint impacts of heavy rainfall and enhanced human activity.
The physical erosion intensity in the mountainous rivers of Taiwan has maintained at a high level and increased steadily in the later stage since ca. 3000 cal yr BP. Such strong physical erosion with steep mountains and fast water flow have exerted a great limitation on regional chemical weathering intensity, making the offshore sediments of Taiwan deposited during the geological past all display moderate chemical alteration. It should be cautious to use the sediment chemistry records offshore of tectonically active mountainous rivers for the interpretation of paleoclimate change in the future. From a global perspective, although mountainous rivers (e.g., Taiwan rivers) have extremely high sediment flux, due to the relatively weak chemical weathering strength, the efficiency of CO2 consumption/transfer from the atmosphere to the hydrosphere through silicate (rock) chemical weathering and the efficiency of bioavailable P conversion from the lithosphere to the hydrosphere through the chemical weathering of detrital apatite are both lower than those in river basins from tectonically stable continent (e.g., the Yangtze River).
In this dissertation, relatively long-term chemical weathering and physical erosion history in small mountainous river basins have been reconstructed, which may provide new insights into the landscape shaping and continent-ocean interactions for river basins in tectonically active mountains. The results highlight the controls of precipitation and human activities on fluvial sediment discharge, although this type of river basin has more favorable conditions for chemical weathering (faster chemical weathering rate), it does not necessarily lead the sediment to experience more intense chemical alteration in such river catchments. The geochemical and morphological characteristics of detrital apatite have been used to distinguish sediment provenance and recover regional weathering and erosion history in the study area for the first time. Detrital is a promising accessory mineral, which will open up new avenues in the study of source-sink process and paleoenvironmental evolution in marginal sea of East Asia.