LATE MIOCENE SEASONAL TO SUBDECADAL CLIMATE VARIABILITY IN THE INDO-WEST PACIFIC (EAST KALIMANTAN, INDONESIA) PRESERVED IN GIANT CLAMS

Two late Miocene Tridacna (giant clam) shells from East Kalimantan (Indonesia) were investigated in order to evaluate their potential as subannually resolved paleoenvironmental archives. Via a combination of X-ray diffraction (XRD), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) trace element analysis, scanning electron microscopy (SEM) and cathodoluminescence (CL) imaging, pristine versus diagenetically altered domains within the shells were identified. LA-ICPMS transects targeting altered aragonite and calcite zones reveal distinct compositional differences in elemental ratios (B/Ca, Mg/Ca, Sr/Ca/Ba/Ca, Mn/Ca, Al/Ca, La/Ca, Ce/Ca) relative to primary shell aragonite. Pristine shell domains are characterized by an intact banding pattern of alternating dark and light growth bands, with which spatially resolved LA-ICPMS element/Ca and micromilled delta O-18 records were aligned. Light delta O-18 values correspond to dark growth bands, indicating growth during warm seasons. The Mg/Ca and/or Sr/Ca ratios covary with oscillating stable oxygen isotope profiles. Progressive increase in Mg/Ca with age demonstrates that besides temperature, growth kinetics exert control over Mg incorporation. If interpreted as temperature controlled only, delta O-18 from both shells represents average seasonal sea-surface temperature (SST) variability of 2.7 +/- 2.1 and 4.6 +/- 1.7 degrees C, respectively. Using published temperature equations and assuming delta O-18(sw) = -0.88 parts per thousand, corresponding mean annual paleo-sea-surface temperatures of 27.8 +/- 0.2 and 28.5 +/- 0.2 degrees C are estimated. Although the fossil Tridacna shells were noticeably affected by alteration on their external surfaces, their internal aragonitic structure is, to a large extent, well preserved. These corresponding paleoproxy records provide detailed insight into tropical SST variability of the Indo-Pacific region during the late Miocene.