Geology topics

Thomas J. Givnish

Publications and source records attributed to Thomas J. Givnish.

4 recordsLinked to original sources

USGS2015

Comment on "Donders, T.H. 2014. Middle Holocene humidity increase in Florida: climate or sea-level? Quaternary Science Reviews 103:170-174."

Donders (2014) has recently proposed that the climate of Florida became progressively wetter over the past 5000 years in response to a marked strengthening of the El Niño regime. This reconstruction is largely based on a re-analysis of pollen records from regions north of Lake Okeechobee ( Fig. 1 ) using a new set of pollen transfer functions. Donders concluded that a latitudinal gradient in precipitation prevailed across Florida since the mid Holocene, but the overall trend was toward progressively wetter conditions from 5000 cal BP to the present. Donders (2014) also proposed that this climatic trend extended across South Florida despite contrary paleo-records from the Everglades. In particular he singled out the Northeast Shark River Slough (NESRS) record of Glaser et al. (2013) as an atypical local signal of paleo-environmental change that was biased by a misinterpretation of the ecology of pine and Amaranthaceae (Amaranth family). In response to this direct critique of our paleo-environmental interpretation, we wish to point out that: Our interpretation of the NESRS sedimentary sequence (site 4, Fig. 1 A) was based on multiple lines of evidence that all indicate a shift from wetter to drier (i.e. less wet) conditions that occurred after 2800 cal BP. A similar climatic shift from wetter to less wet conditions was reported for this time interval from other sites in the Everglades ( Willard et al., 2006 and Willard and Bernhardt, 2011 ) and also from the Caribbean region to the east ( Hodell et al., 1991 , Hodell et al., 1995 and Higuera-Gundy et al., 1999 ). The NESRS site is located in an area where runoff collected from a 10,000 km 2 wetland prior to 1900 AD when most of the drainage from the Everglades was channeled to the sea through the narrow Shark River Slough. The sedimentary sequence at the NESRS site should therefore contain an integrated record of hydrological change across the pre-historical Everglades ( Glaser et al., 2012 ). In addition this essentially non-forested wetland is well suited to accumulate a representative sample of the regional pollen rain in its sediments (e.g. Jaccobson and Bradshaw, 1981 and Prentice, 1985 ).

Florida↗

USGS2013

Holocene dynamics of the Florida Everglades with respect to climate, dustfall, and tropical storms

Aeolian dust is rarely considered an important source for nutrients in large peatlands, which generally develop in moist regions far from the major centers of dust production. As a result, past studies assumed that the Everglades provides a classic example of an originally oligotrophic, P-limited wetland that was subsequently degraded by anthropogenic activities. However, a multiproxy sedimentary record indicates that changes in atmospheric circulation patterns produced an abrupt shift in the hydrology and dust deposition in the Everglades over the past 4,600 y. A wet climatic period with high loadings of aeolian dust prevailed before 2800 cal BP (calibrated years before present) when vegetation typical of a deep slough dominated the principal drainage outlet of the Everglades. This dust was apparently transported from distant source areas, such as the Sahara Desert, by tropical storms according to its elemental chemistry and mineralogy. A drier climatic regime with a steep decline in dustfall persisted after 2800 cal BP maintaining sawgrass vegetation at the coring site as tree islands developed nearby (and pine forests covered adjacent uplands). The marked decline in dustfall was related to corresponding declines in sedimentary phosphorus, organic nitrogen, and organic carbon, suggesting that a close relationship existed between dustfall, primary production, and possibly, vegetation patterning before the 20th century. The climatic change after 2800 cal BP was probably produced by a shift in the Bermuda High to the southeast, shunting tropical storms to the south of Florida into the Gulf of Mexico.

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USGS2012

Carbon and sediment accumulation in the Everglades (USA) during the past 4000 years: rates, drivers, and sources of error

Tropical and sub-tropical wetlands are considered to be globally important sources for greenhouse gases but their capacity to store carbon is presumably limited by warm soil temperatures and high rates of decomposition. Unfortunately, these assumptions can be difficult to test across long timescales because the chronology, cumulative mass, and completeness of a sedimentary profile are often difficult to establish. We therefore made a detailed analysis of a core from the principal drainage outlet of the Everglades of South Florida, to assess these problems and determine the factors that could govern carbon accumulation in this large sub-tropical wetland. Accelerator mass spectroscopy dating provided direct evidence for both hard-water and open-system sources of dating errors, whereas cumulative mass varied depending upon the type of method used. Radiocarbon dates of gastropod shells, nevertheless, seemed to provide a reliable chronology for this core once the hard-water error was quantified and subtracted. Long-term accumulation rates were then calculated to be 12.1 g m -2 yr -1 for carbon, which is less than half the average rate reported for northern and tropical peatlands. Moreover, accumulation rates remained slow and relatively steady for both organic and inorganic strata, and the slow rate of sediment accretion ( 0.2 mm yr -1 ) tracked the correspondingly slow rise in sea level (0.35 mm yr -1 ) reported for South Florida over the past 4000 years. These results suggest that sea level and the local geologic setting may impose long-term constraints on rates of sediment and carbon accumulation in the Everglades and other wetlands.

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USGS2011

Recent and historic drivers of landscape change in the Everglades ridge, slough, and Tree Island mosaic

More than half of the original Everglades extent formed a patterned peat mosaic of elevated ridges, lower and more open sloughs, and tree islands aligned parallel to the dominant flow direction. This ecologically important landscape structure remained in a dynamic equilibrium for millennia prior to rapid degradation over the past century in response to human manipulation of the hydrologic system. Restoration of the patterned landscape structure is one of the primary objectives of the Everglades restoration effort. Recent research has revealed that three main drivers regulated feedbacks that initiated and maintained landscape structure: the spatial and temporal distribution of surface water depths, surface and subsurface flow, and phosphorus supply. Causes of recent degradation include but are not limited to perturbations to these historically important controls; shifts in mineral and sulfate supply may have also contributed to degradation. Restoring predrainage hydrologic conditions will likely preserve remaining landscape pattern structure, provided a sufficient supply of surface water with low nutrient and low total dissolved solids content exists to maintain a rainfall-driven water chemistry. However, because of hysteresis in landscape evolution trajectories, restoration of areas with a fully degraded landscape could require additional human intervention.

Critical Reviews in Environmental Science and Tech↗