Description: The measurement of short-lived 223Ra often involves a second measurement for supported activities, which represents 227Ac in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of 227Ac, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles 227Ac data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of 227Ac near its potential sources. The average 227Ac activities of nearshore marine end-members range from 0.4 dpm/m**3 at the Gulf of Mexico to 3.0 dpm m? 3 in the coastal waters of the Korean Strait. In analogy to 228Ra, we find the extension of adjacent shelf regions to play a substantial role for 227Ac activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean 227Ac inventory of 1.35 * 1018 dpm 227Acex in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm/m**2/y from the deep-sea floor. For the shelf regions, we obtain a global inventory of 227Ac of 4.5 * 10**15 dpm, which cannot be converted directly into a flux value, as the regional loss term of 227Ac to the open ocean would have to be included. Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to 231Paex. Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in 227Ac/228Ra and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including 227Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution.

Description: Sr isotopes in Barite record seawater Sr over the past 35 Ma.

Description: Long-term fire histories provide insight into the effects of climate, ecology and humans on fire activity; they can be generated using accumulation rates of charcoal and soot black carbon in lacustrine sediments. This study uses both charcoal and black carbon, and other paleoclimate indicators from Lake Kinneret (Sea of Galilee), Israel, to reconstruct late Holocene variations in biomass burning and aridity. We compare the fire history data with a regional biomass-burning reconstruction from 18 different charcoal records and with pollen, climate, and population data to decipher the relative impacts of regional climate, vegetation changes, and human activity on fire. We show a long-term decline in fire activity over the past 3070 years, from high biomass burning ~3070-1750 cal yr BP to significantly lower levels after ~1750 cal yr BP. Human modification of the landscape (e.g., forest clearing, agriculture, settlement expansion and early industry) in periods of low to moderate precipitation appears to have been the greatest cause of high biomass burning during the late Holocene in southern Levant, while wetter climate apparently reduced fire activity during periods of both low and high human activity.

Description: The Paleocene-Eocene Thermal Maximum (PETM), ca. 55 Ma, was a period of extreme global warming caused by rapid emission of greenhouse gases. It is unknown what ended this episode of greenhouse warming, but high oceanic export productivity over thousands of years (as indicated by high accumulation rates of barium, Ba) may have been a factor in ending this warm period by carbon sequestration. However, Ba has a short oceanic residence time (~10 k.y.), so a prolonged global increase in Ba accumulation rates requires an increase in input of Ba to the ocean, increasing barite saturation. We use a novel proxy for barite saturation (Sr/Ba in marine barite) to demonstrate that the seawater saturation state with respect to barite did not change across the PETM. The observations of increased barite burial, no change in saturation, and the short residence time can be reconciled if Ba burial decreased at continental margin and shelf sites due to widespread occurrence of suboxic conditions, leading to Ba release into the water column, combined with increased biological export production at some pelagic sites, resulting in Ba sink reorganization.

Description: The strontium isotope ratios (87Sr/86Sr) of marine barite microcrystals separated from Cretaceous sedimentary deposits from Ocean Drilling Program and Deep Sea Drilling Project sites from the Pacific and Indian Oceans have been compared to the composite Sr isotope curve of McArthur et al. The barite in these cores accurately recorded the seawater 87Sr/86Sr ratio, thereby reaffirming the composite Cretaceous strontium curve. Moreover, marine barite is a more reliable recorder of 87Sr/86Sr than is carbonate in sedimentary deposits with high clay content, thereby providing an opportunity for Sr isotope stratigraphy and dating in carbonate-poor or diagenetically altered sections. We have used the barite-derived Sr isotope record to refine the biostratigraphic age models of the sites investigated.

Description: A multiproxy geochemical study of nine sapropel layers from ODP Hole 964A, ODP Leg 160 in the eastern Mediterranean spanning the Pliocene-Holocene time interval provides new insights into the evolution of sapropel deposition. Paleoenvironmental proxies were used for reconstruction of productivity (Ba derived from marine barite), oxygen conditions (trace metal ratios) and sedimentary regime (clay minerals, detrital elements). These proxies reveal a significant increase in river runoff relative to decreasing aeolian input during sapropel deposition over the whole time interval. Ba excess supports the argument that a significant increase in export productivity is the main triggering mechanism for sapropel deposition, although preservation also played an important role. Furthermore, major differences exist in depositional conditions, including both oxygenation and productivity since the Pliocene. Productivity fluctuated substantially and was higher during the Pliocene and Pleistocene than during the Holocene; at the same time decreasing oxygen availability parallels the enhanced productivity. Dysoxic to anoxic conditions appear to coincide with marine productivity maxima, thus suggesting that oxygen depletion may be linked to greater consumption rather than restricted circulation. This correspondence supports the hypothesis that productivity fluctuations resulting from climate oscillations were the main cause of enhanced organic matter contents and also a main controlling factor for reduced oxygen availability.