This research aims to elucidate the Holocene sea level (SL) variability on Minami Daito Island, Japan, where no previous such research has been done due to the apparent absence of any SL indicators. ...Here we use phreatic overgrowths on speleothems (POS) in a cave as an alternative SL indicator. The relationship between the highest modern groundwater fluctuation level (GWFL) and the highest POS level was studied, and the oldest POS calcite was dated using the radiocarbon method considering the dead carbon proportion (the “reservoir effect”). The results show that SL reached its Holocene maximum between ca. 5.1 and 4.6 ka cal BP, after which it remained more or less stable till the present day, with a possible minor sea-level drawdown of ca. 30–35 cm. Although the timing of the maximum Holocene SL coincides with mid-Holocene SL highstand observed across the tropical and subtropical Indo-Pacific, the relative stability of the SL after reaching its peak is not expected in the given subtropical setting. The SL on Minami Daito did not have a distinct highstand in the Holocene and instead followed closely the eustatic sea-level change. These findings are important in better understanding the mechanisms that control local relative SL variability and show the high value of POS as precise SL indicator.
We present bulk average Sr and Mg data for 13 speleothems from different locations in the western equatorial Pacific (Guam, Solomon Islands, and Vanuatu). These data plot on a single straight line in ...a graph of ln(Sr/Ca) vs ln(Mg/Ca) with a slope of ~0.9. A 22,000year record of Sr and Mg in one of these samples from Guam also plots with the same slope, suggesting that the process partitioning Mg and Sr within the Guam speleothem is the same as the one that partitions Mg and Sr between the different Pacific speleothems. We rule out temperature, growth rate, detrital phases, and sea-spray as likely mechanisms for this correlation. We construct mathematical models of limestone diagenesis and show that this cannot explain the slope of the correlation.
Our favoured explanation for the correlation is calcite/water interaction in the form of prior calcite precipitation (PCP) and/or incongruent calcite dissolution (ICD). We present a formal mathematical model of PCP and prove that the slope of a graph of ln(Sr/Ca) and ln(Mg/Ca) is given by KdSr−1KdMg−1. A similar equation is derived for ICD in a companion paper (Sinclair, 2011). Using published values for KdSr and KdMg, this slope is calculated to be ~0.88, in excellent agreement with our observations. Because the slope is independent of solution and host-limestone composition it can be used to diagnose calcite–water interaction in individual cave water studies where host rock composition is unknown, or in speleothems for which no field drip water data or host-rock geochemistry exists. Approximately half of published drip and speleothem Mg+Sr studies plot within error of this slope.
We hypothesize that the overall trend in our Pacific speleothem data results from the individual dripwater systems evolving from a roughly similar initial rock composition set by late Quaternary reef limestone. In the Guam speleothem, the broad peak in Mg and Sr centred on the early–mid Holocene reflects a change in hydrology (rainfall), with the most likely scenario being that dry conditions prevailed on Guam at this time.
► Mg/Ca and Sr/Ca are significantly correlated in a suite of west Pacific speleothems. ► Mathematical models show this is consistent with Prior Calcite Precipitation (PCP). ► Models predict that ln(Sr/Ca) vs ln(Mg/Ca) for PCP has a slope of ~0.88. ► Slope depends only on KdSr and KdMg and may diagnose PCP without drip studies. ► Sr and Mg profiles in a Guam sample suggest dry conditions in the mid Holocene.
The main objective of the study was to produce alternative binder materials, obtained with low cost, low energy consumption, and low CO
production, by regenerating end-of-life (EOL) materials from ...mineral deposits, to replace ordinary Portland cement (OPC). The materials analyzed were ash and slag from the Turceni thermal power plant deposit, Romania. These were initially examined for morphology, mineralogical composition, elemental composition, degree of crystallinity, and heating behavior, to determine their ability to be used as a potential source of supplementary cementitious materials (SCM) and to establish the activation and transformation temperature in the SCM. The in-situ pozzolanic behavior of commercial cement, as well as cement mixtures with different percentages of ash addition, were further observed. The mechanical resistance, water absorption, sorptivity capacity, resistance to alkali reactions (ASR), corrosion resistance, and resistance to reaction with sulfates were evaluated in this study using low-vacuum scanning electron microscopy.
Modern rainwater, cave dripwater and cave stalagmite geochemical time series from a cave in Guam (13°38′N, 144°53′E) are used to better understand how changes in cave stalagmite geochemistry relate ...to aboveground changes in rainfall at a tropical location. A scientific field team based in Guam collects ∼monthly samples from multiple sites for geochemical analyses at a cave and aboveground rainfall from a nearby location. We compute a transfer function between rainfall amount and oxygen isotopic composition (δ18O) of a decrease (increase) of 0.94 ± 0.3 m/year for every 1‰ increase (decrease) in rainfall δ18O, based on data extracted from the International Atomic Energy Agency (IAEA) Global Networks of Isotopes in Precipitation (GNIP) database and from data generated in this study. Dripwater δ18O and Mg/Ca ratios show annual cyclicity at some, but not all sites, accentuating the complex nature of cave hydrology. A stalagmite δ18O record for the last ∼160 years indicates the existence of droughts of decadal length, when rainfall is estimated to be ∼0.65 ± 0.3 m/year less than average conditions. This estimate of rainfall reduction most likely refers to wet season months, as these months preferentially contribute to groundwater recharge. The proxy‐based climate record at Guam provides new evidence highlighting how a rainy site in the Western Pacific Warm Pool today can experience considerable changes in rainfall on decadal timescales.
Key Points
Computed a transfer function bet rainfall amount and oxygen isotopic composition
Dripwater delta18O and Mg/Ca ratios show annual cyclicity at some cave sites
Guam stalagmite delta18O record (past 160 years) shows decadal‐scale droughts
Geomorphic features have been one of the major tools for sea-level change studies. The present work shows an example of sea-level change study on karst terrain in the tropics. Sea-level notches as ...well as flank margin caves were identified in the research area and their elevation measured. The time of formation of the sea-level indicators was constrained by lithology study and dating methods such as facies comparison and U-Th dating. Denudation and uplift were also studied for the same purpose. From this study it can be concluded that sea-level stands within the glacial cycle can cause the formation of flank margin caves and that the position of these sea-level stands can be determined. The research area was estimated to have cumulatively uplifted ∼22 m in the past 125 ka years (∼0.18 mm/yr) while the surface has been denuded some 8 m in the same span of time (∼0.064 mm/yr). Keywords: karst, karst hydrology, flank margin caves, sea-level change, denudation rate, uplift rate, sea-level notches, bioerosional notches, last interglacial, MIS 5e, mid-Holocene sea-level highstand, reef limestone, Guam
Geomorphic features have been one of the major tools for sea-level change studies. The present work shows an example of sea-level change study on karst terrain in the tropics. Sea-level notches as ...well as flank margin caves were identified in the research area and their elevation measured. The time of formation of the sea-level indicators was constrained by lithology study and dating methods such as facies comparison and U-Th dating. Denudation and uplift were also studied for the same purpose. From this study it can be concluded that sea-level stands within the glacial cycle can cause the formation of flank margin caves and that the position of these sea-level stands can be determined. The research area was estimated to have cumulatively uplifted ~22 m in the past 125 ka years (~0.18 mm/yr) while the surface has been denuded some 8 m in the same span of time (~0.064 mm/yr).
