[Show abstract][Hide abstract] ABSTRACT: Stable isotope analysis of leaf waxes in a sediment core from Laguna La Gaiba, a shallow lake located at the Bolivian margin of the Pantanal wetlands, provides new perspective on vegetation and climate change in the lowland interior tropics of South America over the past 40,000 years. The carbon isotopic compositions (δ13C) of long-chain n-alkanes reveal large shifts between C3- and C4-dominated vegetation communities since the last glacial period, consistent with landscape reconstructions generated with pollen data from the same sediment core. Leaf wax δ13C values during the last glacial period reflect an open landscape composed of C4 grasses and C3 herbs from 41–20 ka. A peak in C4 abundance during the Last Glacial Maximum (LGM, ∼21 ka) suggests drier or more seasonal conditions relative to the earlier glacial period, while the development of a C3-dominated forest community after 20 ka points to increased humidity during the last deglaciation. Within the Holocene, large changes in the abundance of C4 vegetation indicate a transition from drier or more seasonal conditions during the early/mid-Holocene to wetter conditions in the late Holocene coincident with increasing austral summer insolation. Strong negative correlations between leaf wax δ13C and values over the entire record indicate that the majority of variability in leaf wax at this site can be explained by variability in the magnitude of biosynthetic fractionation by different vegetation types rather than changes in meteoric water signatures. However, positive deviations from the observed δ13C– trends are consistent with more enriched source water and drier or more seasonal conditions during the early/mid-Holocene and LGM. Overall, our record adds to evidence of varying influence of glacial boundary conditions and orbital forcing on South American Summer Monsoon precipitation in different regions of the South American tropics. Moreover, the relationships between leaf wax stable isotopes and pollen data observed at this site underscore the complementary nature of pollen and leaf wax δ13C data for reconstructing past vegetation changes and the potentially large effects of such changes on leaf wax signatures.
Full-text · Article · Mar 2016 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: Coral growth and carbonate accumulation form the foundation of the coral reef ecosystem. Changes in environmental conditions due to coastal development, climate change, and ocean acidification may pose a threat to net carbonate production in the near future. Controlled laboratory studies demonstrate that calcification by corals and coralline algae is sensitive to changes in aragonite saturation state (Ωa), as well as temperature, light, and nutrition. Studies also show that the dissolution rate of carbonate substrates is impacted by changes in carbonate chemistry. The sensitivity of coral reefs to these parameters must be confirmed and quantified in the natural environment in order to predict how coral reefs will respond to local and global changes, particularly ocean acidification. We estimated the daytime hourly net community metabolic rates, both net community calcification (NCC) and net community productivity (NCP), at Sheltered Reef, an offshore platform reef in the central Red Sea. Average NCC was 8 ± 3 mmol m−2 h−1 in December 2010 and 11 ± 1 mmol m−2 h−1 in May 2011, and NCP was 21 ± 7 mmol m−2 h−1 in December 2010 and 44 ± 4 mmol m−2 h−1 in May 2011. We also monitored a suite of physical and chemical properties to help relate the rates at Sheltered Reef to published rates from other sites. While previous research shows that short-term field studies investigating the NCC–Ωa relationship have differing results due to confounding factors, it is important to continue estimating NCC in different places, seasons, and years, in order to monitor changes in NCC versus Ω in space and time, and to ultimately resolve a broader understanding of this relationship.
[Show abstract][Hide abstract] ABSTRACT: North Atlantic late Pleistocene climate (60,000 to 11,650 years ago) was characterized by abrupt and extreme millennial duration oscillations known as Dansgaard-Oeschger (D-O) events. However, during the Last Glacial Maximum (LGM) 23,000 to 19,000 cal years ago (23 to 19 ka), no D-O events are observed in the Greenland ice cores. Our new analysis of the Greenland δ18O record reveals a switch in the stability of the climate system around 30 ka, suggesting that a critical threshold was passed. Climate system modeling suggests that low axial obliquity at this time caused vastly expanded sea ice in the Labrador Sea, shifting Northern Hemisphere westerly winds south and reducing the strength of meridional overturning circulation. The results suggest that these feedbacks tipped the climate system into full glacial conditions, leading to maximum continental ice growth during
Full-text · Article · Dec 2015 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: The mechanisms of Late Pleistocene megafauna extinctions remain fiercely contested, with human impact or climate change cited as principal drivers. We compared ancient DNA and radiocarbon data from 31 detailed time series of regional megafaunal extinctions and replacements over the past 56,000 years with standard and new combined records of Northern Hemisphere climate in the Late Pleistocene. Unexpectedly, rapid climate changes associated with interstadial warming events are strongly associated with the regional replacement or extinction of major genetic clades or species of megafauna. The presence of many cryptic biotic transitions before the Pleistocene/Holocene boundary revealed by ancient DNA confirms the importance of climate change in megafaunal population extinctions and suggests that metapopulation structures necessary to survive such repeated and rapid climatic shifts were susceptible to human impacts.
[Show abstract][Hide abstract] ABSTRACT: X-ray fluorescence (XRF) scanning of sediment cores allows the rapid acquisition of bulk geochemical data at high resolution. The XRF core scanner data, which are expressed as elemental counts or peak areas, are mainly related to elemental concentrations, but they are also influenced by a series of sediment physical properties that vary with depth, such as bulk density, water content, organic matter content, and grain size. Here, we investigate the influence of grain size on elemental XRF peak areas by comparing ITRAX XRF core scanner measurements to ICP-AES elemental concentrations for two sediment cores with variable grain size. Results provide evidence for a limited influence of sediment grain size on XRF peak areas. This influence is negligible for sediment cores with grain-size variations of 10 μm or less. Our data also demonstrate that for cores with large grain-size variations, correcting the peak areas for water content improves the precision of the XRF measurements by a factor of three. This study therefore demonstrates that, for most sediment cores, the precision of data obtained by XRF core scanning is not significantly altered by grain-size variations.
[Show abstract][Hide abstract] ABSTRACT: The oceans absorb anthropogenic CO2 from the atmosphere, lowering surface ocean pH, a concern for calcifying marine organisms. The impact of ocean acidification is challenging to predict as each species appears to respond differently and because our knowledge of natural changes to ocean pH is limited in both time and space. Here we reconstruct 222 years of biennial seawater pH variability in the Sargasso Sea from a brain coral, Diploria labyrinthiformis. Using hydrographic data from the Bermuda Atlantic Time-series Study and the coral-derived pH record, we are able to differentiate pH changes due to surface temperature versus those from ocean circulation and biogeochemical changes. We find that ocean pH does not simply reflect atmospheric CO2 trends but rather that circulation/biogeochemical changes account for >90% of pH variability in the Sargasso Sea and more variability in the last century than would be predicted from anthropogenic uptake of CO2 alone.
Full-text · Article · May 2015 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: During the African Humid Period about 14,800 to 5,500 years ago, changes in incoming solar radiation during Northern Hemisphere summers led to the large-scale expansion and subsequent collapse of the African monsoon. Hydrologic reconstructions from arid North Africa show an abrupt onset and termination of the African Humid Period. These abrupt transitions have been invoked in arguments that the African monsoon responds rapidly to gradual forcing as a result of nonlinear land surface feedbacks. Here we present a reconstruction of precipitation in humid tropical West Africa for the past 20,000 years using the hydrogen isotope composition of leaf waxes preserved in sediments from Lake Bosumtwi, Ghana. We show that over much of tropical and subtropical Africa the monsoon responded synchronously and predictably to glacial reorganizations of overturning circulation in the Atlantic Ocean, but the response to the relatively weaker radiative forcing during the African Humid Period was more spatially and temporally complex. A synthesis of hydrologic reconstructions from across Africa shows that the termination of the African Humid Period was locally abrupt, but occurred progressively later at lower latitudes. We propose that this time-transgressive termination of the African Humid Period reflects declining rainfall intensity induced directly by decreasing summer insolation as well as the gradual southward migration of the tropical rainbelt that occurred during this interval.
No preview · Article · Jan 2015 · Nature Geoscience
[Show abstract][Hide abstract] ABSTRACT: A record of the hydrogen isotopic composition of terrestrial leaf waxes ( ) in sediment cores from Lake Titicaca provides new insight into the precipitation history of the Central Andes and controls of South American Summer Monsoon (SASM) variability since the last glacial period. Comparison of the record with a 19-kyr record from the nearby Illimani ice core supports the interpretation that precipitation is the primary control on with a lesser but significant role for local evapotranspiration and other secondary influences on . The Titicaca record confirms overall wetter conditions in the Central Andes during the last glacial period relative to a drier Holocene. During the last deglaciation, abrupt shifts correspond to millennial-scale events observed in the high-latitude North Atlantic, with dry conditions corresponding to the Bølling-Allerød and early Holocene periods and wetter conditions during late glacial and Younger Dryas intervals. We observe a trend of increasing monsoonal precipitation from the early to the late Holocene, consistent with summer insolation forcing of the SASM, but similar hydrologic variability on precessional timescales is not apparent during the last glacial period. Overall, this study demonstrates the relative importance of high-latitude versus tropical forcing as a dominant control on glacial SASM precipitation variability.
No preview · Article · Dec 2014 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: Global climate change and anthropogenic activities are threatening the future survival
of coral reef ecosystems. The ability of reef-building zooxanthellate coral to survive these
stressors may be determined through fundamental differences within their symbiotic dinoflagellates
(Symbiodinium sp.). We define the in vitro apoptotic response of 2 evolutionarily distant Symbiodinium
sp., subtypes B2 and C1, to determine the synergistic effects of disease and temperature
on cell viability using flow cytometry. The putative yellow band disease (YBD) consortium of
Vibrio spp. bacteria and temperature (33°C) had a positive synergistic effect on C1 apoptosis,
while B2 displayed increased apoptosis to elevated temperature (29 and 33°C), the Vibrio consortium,
and a lone virulent strain of V. alginolyticus, but no synergistic effects. Additionally, heat
shock protein 60 expression revealed differential cell-mediated temperature sensitivity between
subtypes via western blotting. This result marks the first evidence of Symbiodinium sp. apoptotic
variations to YBD pathogens and emphasizes the potential impact of synergistic stress on globally
distributed coral−Symbiodinium symbioses.
Full-text · Article · Dec 2014 · Diseases of Aquatic Organisms
[Show abstract][Hide abstract] ABSTRACT: The climate of Chilean Patagonia is strongly influenced by the southern westerlies, which control the amount and latitudinal distribution of precipitation in the southern Andes. In austral summer, the Southern Westerly Wind Belt (SWWB) is restricted to the high latitudes. It expands northward in winter, which results in a strong precipitation seasonality between ∼35 and 45°S. Here, we present a new precipitation seasonality proxy record from Quitralco fjord (46°S), where relatively small latitudinal shifts of the SWWB result in large changes in precipitation seasonality. Our 1400 yr record is based on sedimentological and geochemical data obtained on a sediment core collected in front of a small river that drains the Patagonian Andes, which makes this site particularly sensitive to changes in river discharge. Our results indicate Fe/Al and Ti/Al values that are low between 600 and 1200 CE, increasing at 1200-1500 CE, and high between 1500 and 1950 CE. Increasing Fe/Al and Ti/Al values reflect a decrease in mean sediment grain-size from 30 to 20 μm, which is interpreted as a decrease in seasonal floods resulting from an equatorward shift of the SWWB. Our results suggest that, compared to present-day conditions, the SWWB was located in a more poleward position before 1200 CE. It gradually shifted towards the equator in 1200-1500 CE, where it remained in a sustained position until 1950 CE. This pattern is consistent with most precipitation records from central and southern Chile. The comparison of our record with published regional sea surface temperature (SST) reconstructions for the late Holocene shows that equatorward shifts of the SWWB are systematically coeval with decreasing SSTs and vice versa, which resembles fluctuations over glacial-interglacial timescales. We argue that the synchronicity between SST and SWWB changes during the last 1400 years represents the response of the SWWB to temperature changes in the Southern Hemisphere.
Full-text · Article · Dec 2014 · Quaternary Science Reviews
[Show abstract][Hide abstract] ABSTRACT: As sea surface temperatures rise and the global human population increases, large-scale field observations of marine organism health and water quality are increasingly necessary. We investigated the health of corals from the family Fungiidae using visual observations in relation to water quality and microbial biogeochemistry parameters along 1300 km of the Red Sea coast of Saudi Arabia. At large scales, incidence of lesions caused by unidentified etiology showed consistent signs, increasing significantly from the northern to southern coast and positively correlated to annual mean seawater temperatures. Lesion abundance also increased to a maximum of 96% near the populous city of Jeddah. The presence of lesioned corals in the region surrounding Jeddah was strongly correlated with elevated concentrations of ammonium and changes in microbial communities that are linked to decreased water quality. This study suggests that both high seawater temperatures and nutrient pollution may play an indirect role in the formation of lesions on corals.
No preview · Article · Jul 2014 · Marine Environmental Research
[Show abstract][Hide abstract] ABSTRACT: Paired strontium-to-calcium (Sr/Ca) and δ18O measurements for two Porites lutea corals recovered from Hon Tre Island, Vietnam, are strongly correlated to sea surface temperature (SST) and precipitation at monthly to interannual time-scales. Least squares linear regression of monthly Sr/Ca to SST shows a strong, significant correlation (r2 = 0.77, p < .0001), with root mean square residuals of 0.9 °C. 3-year averaged (binned) Sr/Ca for wet (Sep–Nov) and dry (Jan–Mar) seasons separately captures SST variability at interannual time scales (Sr/Ca RMSR = 0.42 °C and 0.70 °C for wet and dry seasons, respectively). Coral δ18O correlates weakly to SST at seasonal and interannual time scales for wet and dry seasons, with significant anomalies (δ18O RMSR = 2.4 °C and 1.65 °C, respectively). Correcting the SST influence on coral δ18O using paired Sr/Ca values provides estimates of δ18O of seawater (δ18Osw). 3-year averaged δ18Osw during the wet season shows a significant correlation to local precipitation (r2 = 0.54, p = 0.01). These results show that coral Sr/Ca in this location accurately reflects SST at a number of timescales, and that seawater δ18O composition in the wet season is controlled by local precipitation, largely unmodified by ocean circulation during the winter monsoon. This study highlights the sensitivity and utility of coral geochemistry in this region for reliably reconstructing SST and monsoonal precipitation.
Full-text · Article · Jun 2014 · Palaeogeography Palaeoclimatology Palaeoecology