Tremaine D.M.,Florida State University |
Froelich P.N.,Froelich Education Services
Geochimica et Cosmochimica Acta | Year: 2013
Trace element variations in ancient cave speleothems are often interpreted as indicators of changes in paleo-rainfall and hydrologic conditions. However, these records are difficult to interpret without an understanding of the physicochemical controls on stalagmite chemistry plus site-specific calibration of changes in net rainfall to variations in dripwater and speleothem chemistry. In this study we examine geochemical relationships between net rainfall (Precipitation minus Evapotranspiration; P-ET), drip rates, drip water chemistry, and contemporaneous calcite chemistry to test the hypothesis that speleothem Mg/Ca and Sr/Ca records are proxies for rainfall amount. HRC is contained within four low-magnesium limestone units capped sporadically by a remnant dolomitic limestone. Aqueous concentrations of magnesium (post evapotranspiration) decrease with increasing vertical travel distance between the soil zone and the point of in-cave drip emergence ( Drip Path Length - DPL) as dissolved high-Mg solutions sourced from the dolomitic caprock are diluted with dissolved low-Mg limestone waters sourced from the host limestone. Dripwater Mg/Ca and Sr/Ca ratios covary and provide diagnostic indicators of the two dominant mechanisms controlling dripwater chemistry: (1) mixing of post-evaporative solutions derived from two geochemical endmembers (dissolution of dolomite and limestone); and (2) evolution of hydrochemistry away from dissolved bedrock compositions due to Prior Calcite Precipitation (PCP) above the drip sites. By resolving the linear mixing relationships for drip water Mg/Ca and Sr/Ca sources and the distribution coefficients for trace element transfer in the PCP dripwater-to-calcite precipitation reactions and applying these principles to our time series, we find that the extent of PCP production within the karst is directly controlled by the balance between Precipitation (P) and Evapotranspiration (ET): higher net rainfall (P-ET > 1: wet conditions) reduces PCP, and lower net rainfall with increased evapotranspiration (P-ET. <. 1) increases PCP. Farmed calcite X/Ca ratios faithfully track hydrologically-influenced seasonal variations in dripwater chemistry for X=Mg, Sr, and Na. However, the relationship between changes in net rainfall and changes in Mg/Ca and Sr/Ca ratios in modern calcite is unique at each site and differs significantly at closely-spaced drip/stalagmite locations. This suggests that in situ modern hydrochemical calibrations should be performed atop individual speleothems prior to harvesting for paleoclimate investigations, and that such calibrations may not reflect past conditions as drip paths change. We apply this understanding to published dripwater data and speleothem time series from other caves. A major implication is that in order to interpret stalagmite Sr/Ca and Mg/Ca ratios as 'wet vs. dry' proxies, speleothem Sr/Ca and Mg/Ca variations must be coherent and in-phase over all time periods (i.e., Sr/Mg ratios must be constant). These criteria will help to distinguish 'rainfall amount' versus 'rainfall source' in speleothem δ18O records. © 2013 Elsevier Ltd.
Tremaine D.M.,Florida State University |
Froelich P.N.,Froelich Education Services |
Wang Y.,Florida State University
Geochimica et Cosmochimica Acta | Year: 2011
Understanding the relationships between speleothem stable isotopes (δ13C δ18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave.Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2ms-1 in winter and 0.4ms-1 in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO2. A clear relationship is found between calcite δ13C and cave air ventilation rates estimated by proxies pCO2 and 222Rn. Calcite δ13C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ13CCaCO3=-7‰. A whole-cave " Hendy test" at multiple contemporaneous farming sites reveals that ventilation induces a +1.9±0.96‰ δ13C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the " Hendy test" has implications for interpreting δ13C records in ancient speleothems. Calcite δ13CCaCO3 may be a proxy not only for atmospheric CO2 or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows.Farmed calcite δ18O was found to exhibit a +0.82±0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ13CCaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ18OCaCO3 appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water-calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water-calcite oxygen isotope fractionation across a range of temperatures and cave environments: 1000lnα=16.1103T-1-24.6 with a fractionation temperature dependence of Δδ18O/ΔT=-0.177‰/°C, lower than the currently accepted -0.206‰/°C. © 2011 Elsevier Ltd.
Peterson R.N.,Florida State University |
Peterson R.N.,Coastal Carolina University |
Burnett W.C.,Florida State University |
Opsahl S.P.,Joseph W. Jones Ecological Research Center |
And 6 more authors.
Journal of Environmental Radioactivity | Year: 2013
Suspended particles in rivers can carry metals, nutrients, and pollutants downstream which can become bioactive in estuaries and coastal marine waters. In river systems with multiple sources of both suspended particles and contamination sources, it is important to assess the hydrologic conditions under which contaminated particles can be delivered to downstream ecosystems. The Apalachicola-Chattahoochee-Flint (ACF) River system in the southeastern United States represents an ideal system to study these hydrologic impacts on particle transport through a heavily-impacted river (the Chattahoochee River) and one much less impacted by anthropogenic activities (the Flint River). We demonstrate here the utility of natural radioisotopes as tracers of suspended particles through the ACF system, where particles contaminated with arsenic (As) and antimony (Sb) have been shown to be contributed from coal-fired power plants along the Chattahoochee River, and have elevated concentrations in the surficial sediments of the Apalachicola Bay Delta. Radium isotopes (228Ra and 226Ra) on suspended particles should vary throughout the different geologic provinces of this river system, allowing differentiation of the relative contributions of the Chattahoochee and Flint Rivers to the suspended load delivered to Lake Seminole, the Apalachicola River, and ultimately to Apalachicola Bay. We also use various geochemical proxies (40K, organic carbon, and calcium) to assess the relative composition of suspended particles (lithogenic, organic, and carbonate fractions, respectively) under a range of hydrologic conditions. During low (base) flow conditions, the Flint River contributed 70% of the suspended particle load to both the Apalachicola River and the bay, whereas the Chattahoochee River became the dominant source during higher discharge, contributing 80% of the suspended load to the Apalachicola River and 62% of the particles entering the estuary. Neither of these hydrologic scenarios, which were moderately low flow regimes, appeared to transport particles contaminated with arsenic and antimony to Apalachicola Bay. © 2012 Elsevier Ltd.
Froelich F.,Froelich Education Services |
Misra S.,University of Cambridge
Oceanography | Year: 2014
Hothouse climates in Earth's geologic past, such as the Eocene epoch, are thought to have been caused by the release of large amounts of carbon dioxide and/or methane, which had been stored as carbon in biogenic gases and organic matter in sediments, to the ocean-atmosphere system. However, to avoid runaway temperatures, there must be long-term negative feedbacks that consume CO2 on time scales longer than the ~ 100,000 years generally ascribed to ocean uptake of CO2 and burial of marine organic carbon. Here, we argue that continental chemical weathering of silicate rocks, the ultimate long-term (multi-million year) sink for CO2, must have been almost dormant during the late Paleocene and early Eocene, allowing buildup of atmospheric CO2 to levels exceeding 1000 ppm. This reduction in the strength of the CO2 sink was the result of minimal global tectonic uplift of silicate rocks that did not produce mountains susceptible to physical and chemical weathering, an inversion of the Uplift-Weathering Hypothesis. There is lack of terrestrial evidence for absence of uplift; however, the δ7Li chemistry of the Paleogene ocean indicates that continental relief during this period of the Early Cenozoic was one of peneplained (flat) continents characterized by high chemical weathering intensity and slow physical and chemical weathering rates, yielding low river fluxes of suspended solids, dissolved cations, and clays delivered to the sea. Only upon re-initiation of mountain building in the Oligocene-Miocene (Himalayas, Andes, Rockies) and drifting of these continental blocks to low-latitude locations near the Inter-Tropical Convergence Zone and monsoonal climate belts did continental weathering take on modern characteristics of rivers with high suspended loads and incongruent weathering, with much of the cations released during weathering being sequestered into secondary clay minerals. The δ7Li record of the Cenozoic ocean provides another piece of circumstantial evidence in support of the Late Cenozoic Uplift-Weathering Hypothesis. © 2014 by The Oceanography Society. All rights reserved.