UC Santa Barbara

Recent studies along the northwestern Gulf of Mexico document rapid back-stepping of estuarine environments of up to 20 km and 150 m/yr at ca. 8.2 thousand years ago (ka), 4.8, ka, and 2.6 ka. If such rapid changes in coastal environments occurred today along the urbanized coast of the Gulf of Mexico major economical and ecological loss would occur. Of these three backstepping events, only one can be tied to a known cause – the 8.2 ka event related to a rapid increase in the rate of relative sea-level rise. However, the cause of the latter two, the 4.8 and 2.6 ka events are largely unknown. To determine the relative roles of changes in sea level and climate in these two events, paleo sea-level (Chapter 2), climate (Chapter 3), and stratigraphic records (Chapter 4) are presented from upper Baffin Bay, Texas and neighboring playa Laguna Salada.

As commonly used high-resolution sea-level indicators are absent from the southern Texas Coast, a new sea-level indicator is needed. I test the use of intertidal microbial mats in reconstructing Holocene sea levels from Baffin, Bay Texas (Chapter 2). The indicative range of microbial mats is ±0.29 m, much less than the ±2 m indicative range of typical sea-level indicators currently used along the semiarid Texas coast. The elevations of 22 buried radiocarbon-dated microbial mats plot within error of relative sea-level data derived from the central Texas coast for the past 5.0 ky indicating that microbial mats provide a robust proxy for paleo–sea levels along semiarid and arid coastlines. The high-resolution microbial mat relative sea-level record derived from the tectonically stable central Texas coast also provides a test of recently proposed Holocene Highstands based on emergent coastal deposits in Texas and Alabama. The microbial mat relative sea-level data, similar to relative sea-level data derived from Mississippi Delta basal peats and lower-resolution Texas estuarine bivalves, indicates an ever-decreasing rate of relative sea-level rise since ca. 5.0 ka and precludes the possibility of Holocene Highstands in the northern Gulf of Mexico.

Paleoclimate records are also sparse along the southern Texas coast. A new quantitative drought proxy is derived from a transfer function between X-Ray Fluorescence (XRF) elemental data from a Texas playa and a tree-ring drought record (Chapter 3). Using this transfer function, a 954-year tree-ring drought record was extended to ca. 3,000 ka. Ba, Br, and Pb were utilized as predictor variables. Machine learning algorithms, utilized to derive the transfer function, had maximum validation accuracies of 94%. Changes in the extended drought record correspond with the timing of the Roman Climate Optimum, Medieval Warm Period, Little Ice Age, and changes in North Atlantic sea surface temperatures (SST). Increased drought frequency is coeval with nearby dune migration ca. 0.2 ka, 1.9 ka, and 2.6 ka. The highest drought frequency in the record occurs during the Medieval Warm Period ca. 1.0 ka followed by a decrease in drought frequency during the Little Ice Age ca. 0.4 ka. Increased drought frequency accompanies increased North Atlantic SST since 3 ka. This trend of warm North Atlantic SST and dry conditions over the study area follows secular meteorological observations and tree-ring records. These results indicate that lacustrine derived XRF element data can be used as a quantitative tool to reconstruct past drought records, and that North Atlantic SST modulated drought in southern Texas for the last 3,000 years.

Within Baffin Bay, five flooding surfaces making periods of abrupt environmental change, between 1.1 - 1.0 ka, 2.7 - 2.1 ka, 3.8 - 3.0 ka, 5.2 – 4.9 ka, and 6.5 – 5.7 ka occur through a time-period of ever decreasing rates of relative sea-level rise and within error of periods of drying in southern Texas at ca. 1.0 ka, 2.6 ka, 3.4 ka, 4.8 ka, and 5.5 ka (Chapter 4). I hypothesize that these flooding surfaces, occurring when sea level in the Gulf of Mexico was rising < 2 mm/yr, and during independently documented drying events, were primarily driven by changes in climate through declines in fluvial sediment supply to the coast.