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  • 1
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    Assessing sedimentary records of paleohurricane activity using modeled hurricane climatology (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q09V10, doi:10.1029/2008GC002043.
    Description: Patterns of overwash deposition observed within back-barrier sediment archives can indicate past changes in tropical cyclone activity; however, it is necessary to evaluate the significance of observed trends in the context of the full range of variability under modern climate conditions. Here we present a method for assessing the statistical significance of patterns observed within a sedimentary hurricane-overwash reconstruction. To alleviate restrictions associated with the limited number of historical hurricanes affecting a specific site, we apply a recently published technique for generating a large number of synthetic storms using a coupled ocean-atmosphere hurricane model set to simulate modern climatology. Thousands of overwash records are generated for a site using a random draw of these synthetic hurricanes, a prescribed threshold for overwash, and a specified temporal resolution based on sedimentation rates observed at a particular site. As a test case we apply this Monte Carlo technique to a hurricane-induced overwash reconstruction developed from Laguna Playa Grande (LPG), a coastal lagoon located on the island of Vieques, Puerto Rico in the northeastern Caribbean. Apparent overwash rates in the LPG overwash record are observed to be four times lower between 2500 and 1000 years B.P. when compared to apparent overwash rates during the last 300 years. However, probability distributions based on Monte Carlo simulations indicate that as much as 65% of this drop can be explained by a reduction in the temporal resolution for older sediments due to a decrease in sedimentation rates. Periods of no apparent overwash activity at LPG between 2500 and 3600 years B.P. and 500–1000 years B.P. are exceptionally long and are unlikely to occur (above 99% confidence) under the current climate conditions. In addition, breaks in activity are difficult to produce even when the hurricane model is forced to a constant El Niño state. Results from this study continue to support the interpretation that the western North Atlantic has exhibited significant changes in hurricane climatology over the last 5500 years.
    Description: Funding for this research was provided by the Earth Systems History Program of the National Science Foundation, Risk Prediction Initiative, National Geographic Society, Coastal Ocean Institute at WHOI, and the Andrew W. Mellon Foundation Endowed Fund for Innovative Research.
    Keywords: Tropical cyclones ; Paleotempestology ; Paleoclimate ; Holocene ; Climate change ; Sedimentology
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Late Holocene variability in Florida Current surface density : patterns and possible causes (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA4001, doi:10.1029/2004PA001008.
    Description: Planktonic foraminiferal δ18O time series from three well-dated, high sedimentation rate cores near the Florida Keys (24.4°N, 83.3°W) exhibit repeated centennial to millennial-scale oscillations during the late Holocene. Isotopic shifts of 0.2–0.3‰ over the past 5200 years represent changes in sea-surface temperature (SST) of 1.0–1.5°C or salinity variability of 1–2 psu. The largest significant isotopic events are centered at approximately 200, 2000, 3200, and prior to 4000 calendar years BP. High Florida Current δ18O during the Little Ice Age (LIA) correlates with published records of high δ18O in the Sargasso Sea and low SST off the coast of west Africa. An interval of generally low δ18O in the Florida Straits from 1800 to 500 years BP is synchronous with the Medieval Warm Period off west Africa but leads low δ18O in the Sargasso Sea by several hundred years. Synchronous cooling across the subtropical gyre during the LIA is difficult to explain using interannual North Atlantic Oscillation patterns but may be consistent with the simulated effects of reduced solar irradiance. At frequencies between 1/1000 and 1/300 years during the Late Holocene, Florida Current δ18O is coherent with a published estimate of 14C production rate. Radiocarbon production seems to lead δ18O at these frequencies, but uncertainty in the phase calculation precludes a clear lead-lag relationship. At frequencies lower than 1/300 years, Florida Current δ18O is coherent and in phase with atmospheric Δ14C. The coherence of Δ14C and δ18O at periods 〉1000 years implies oceanic circulation may play a role in modulating atmospheric radiocarbon on millennial timescales.
    Description: This work was supported by NSF grants OCE-9905605 and OCE-0096469.
    Keywords: Holocene ; Florida current ; Density
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  • 3
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    Ancient DNA derived from alkenone-biosynthesizing haptophytes and other algae in Holocene sediments from the Black Sea (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 21 (2006): PA1005, doi:10.1029/2005PA001188.
    Description: Holocene sea surface temperatures (SST) of the Black Sea have been reconstructed using sedimentary C37 unsaturated alkenones assumed to be derived from the coccolithophorid haptophyte Emiliania huxleyi, whose fossil coccoliths are an important constituent of the unit I sediments. However, alkenones can also be biosynthesized by haptophyte species that do not produce microscopic recognizable coccoliths. A species-specific identification of haptophytes is important in such U 37 K′-based past SST reconstructions since different species have different alkenone-SST calibrations. We showed that 18S rDNA of E. huxleyi made up only a very small percentage (less than 0.8%) of the total eukaryotic 18S rDNA within the up to 3600-year-old fossil record obtained from the depocenter (〉2000 m) of the Black Sea. The predominant fossil 18S rDNA was derived from dinoflagellates (Gymnodinium spp.), which are predominant members of the summer phytoplankton bloom in the modern Black Sea. Using a polymerase chain reaction/denaturing gradient gel electrophoresis method selective for haptophytes, we recovered substantial numbers of a preserved 458-base-pair (bp)-long 18S rDNA fragment of E. huxleyi from the Holocene Black Sea sediments. Additional fossil haptophyte sequences were not detected, indicating that the E. huxleyi alkenone-SST calibration can be applied for at least the last ∼3600 years. The ancient E. huxleyi DNA was well protected against degradation since the DNA/alkenone ratio did not significantly decrease throughout the whole sediment core and 20% of ∼2700-year-old fossil E. huxleyi DNA was still up to 23,000 base pairs long. We showed that fossil DNA offers great potential to study the Holocene paleoecology and paleoenvironment of anoxic deep-sea settings in unprecedented detail.
    Description: This work was supported by a grant from the Netherlands Organization for Scientific Research (NWO) (Open Competition Program 813.13.001 to M.J.L.C.) and NSF grant OCE0117824 to S.G.W., which we greatly appreciate.
    Keywords: Fossil DNA ; DGGE ; Paleoecology ; Holocene ; Black Sea ; Ancient haptophytes
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Florida Straits density structure and transport over the last 8000 years (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA3209, doi:10.1029/2008PA001717.
    Description: The density structure across the Florida Straits is reconstructed for the last 8000 years from oxygen isotope measurements on foraminifera in sediment cores. The oxygen isotope measurements suggest that the density contrast across the Florida Current increased over this time period. The magnitude of this change corresponds to an increase in the geostrophic transport referenced to 800 m water depth of 4 sverdrups (Sv) over the last 8000 years. The spatial and seasonal distribution of incoming solar radiation due to changes in the Earth's orbit has caused systematic changes in the atmospheric circulation, including a southward migration of the Intertropical Convergence Zone over the last 8000 years. These changes in atmospheric circulation and the associated wind-driven currents of the upper ocean could readily account for a 4 Sv increase in the strength of the Florida Current. We see no evidence in our data for dramatic changes in the strength of the Atlantic Meridional Overturning Circulation over this time period.
    Description: This work was supported by NSF grants OCE-9984989/OCE-0428803 and OCE-0096472 to J.L.-S. and NSF grants OCE-0096469 to W.B.C.
    Keywords: Florida Current ; Holocene ; Foraminifera
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Rapid sea level rise and ice sheet response to 8,200-year climate event (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 34 (2007): L20603, doi:10.1029/2007GL031318.
    Description: The largest abrupt climatic reversal of the Holocene interglacial, the cooling event 8.6–8.2 thousand years ago (ka), was probably caused by catastrophic release of glacial Lake Agassiz-Ojibway, which slowed Atlantic meridional overturning circulation (AMOC) and cooled global climate. Geophysical surveys and sediment cores from Chesapeake Bay reveal the pattern of sea level rise during this event. Sea level rose ~14 m between 9.5 to 7.5 ka, a pattern consistent with coral records and the ICE-5G glacio-isostatic adjustment model. There were two distinct periods at ~8.9–8.8 and ~8.2–7.6 ka when Chesapeake marshes were drown as sea level rose rapidly at least ~12 mm yr−1. The latter event occurred after the 8.6–8.2 ka cooling event, coincided with extreme warming and vigorous AMOC centered on 7.9 ka, and may have been due to Antarctic Ice Sheet decay.
    Description: Cronin, Willard, Thunell, Berke supported by USGS Earth Surface Dynamics Program; Vogt and Pohlman by Office of Naval Research; Halka by MGS.
    Keywords: Sea-level rise ; Holocene ; 8.2 ka event
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Seawater isotope constraints on tropical hydrology during the Holocene (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 34 (2007): L13701, doi:10.1029/2007GL030017.
    Description: Paleoceanographic data from the low latitude Pacific Ocean provides evidence of changes in the freshwater budget and redistribution of freshwater within the basin during the Holocene. Reconstructed Holocene seawater δ 18O changes compare favorably to differences predicted between climate simulations for the middle Holocene (MH) and for the pre-Industrial late Holocene (LH). The model simulations demonstrate that changes in the tropical hydrologic cycle affect the relationship between δ 18Osw and surface salinity, and allow, for the first time, quantitative estimates of western Pacific salinity change during the Holocene. The simulations suggest that during the MH, the mean salinity of the Pacific was higher because less water vapor was transported from the Atlantic Ocean and more was transported to the Indian Ocean. The salinity of the western Pacific was enhanced further due both to the greater advection of salt to the region by ocean currents and to an increase in continental precipitation at the expense of maritime precipitation, the latter a consequence of the stronger Asian summer monsoon.
    Description: This work was supported by NSF grants ATM-0501241, ATM-0501351, and WHOI’s Ocean and Climate Change Institute.
    Keywords: Holocene ; Tropical Pacific ; Hydrology ; Paleoceanography ; Geochemical tracers ; Insolation forcing
    Repository Name: Woods Hole Open Access Server
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