A 2,500-year record of environmental change in Highlands Hammock State Park (Central Florida, USA) inferred from siliceous microfossils

Analysis of siliceous microfossils of a 79 cm long peat sediment core from Highlands Hammock State Park, Florida, revealed distinct changes in the local hydrology during the past 2,500 years. The coring site is a seasonally inundated forest where water availability is directly influenced by precipitation. Diatoms, chrysophyte statospores, sponge remains and phytoliths were counted in 25 samples throughout the core. Based on the relative abundance of diatom species, the record was subdivided into four diatom assemblage zones, which mainly reflect the hydrological state of the study site. An age-depth relationship based on radiocarbon measurements of eight samples reveals a basal age of the core of approximately 2,500 cal. yrs. BP. Two significant changes of diatom assemblage composition were found that could be linked to both, natural and anthropogenic influences. At 700 cal. yrs. BP, the diatom record documents a shift from tychoplanktonic Aulacoseira species to epiphytic Eunotia species, indicating a shortening of the hydroperiod, i.e. the time period during which a wetland is covered by water. This transition was interpreted as being triggered by natural climate change. In the middle of the twentieth century a second major turnover took place, at that time however, as a result of human impact on the park hydrology through the construction of dams and canals close to the study site

Marine connections of Amazonia: Evidence from foraminifera and dinoflagellate cysts (early to middle Miocene, Colombia/Peru)

Species composition in the present-day Amazonian heartland has an imprint of past marine influence. The exact nature, timing and extent of this marine influence, however, are largely unresolved. Here we use calcareous tests of foraminifera and marine palynomorphs from Miocene sediments in northwestern Amazonia to extend on current estimates for salinity ranges, paleoenvironments and paleogeography. Our samples mostly contain tests and/or organic linings of euryhaline (tolerant to a wide range of salinity) foraminifera of the genera Ammonia, Trochammina and Elphidium, with Ammonia being by far the dominant genus at all locations. Organic-walled dinoflagellate cysts (dinocysts), such as Spiniferites, Brigantedinium and Tuberculodinium vancampoae, are also common at a number of sites. This association of foraminifera and dinocyst taxa points at varying salinities, with aberrant forms of Ammonia indicating lower limits of 0-10 psu (practical salinity units) whereas dinocyst associations suggest more marine conditions. Such regional heterogeneity is common at the interface of shallow marine to freshwater environments, like estuaries. We conclude that during the early and middle Miocene marginal marine conditions reached at least 2000 km inland from the Caribbean portal. Global high sea level and fast subsidence in the sub-Andean zone are thought to be the controlling mechanisms of the marine incursions. Lowering of global sea level and a change in tectonic regime terminated the incursions in the course of the Plio-Pleistocene

Biota-hydrology interactions during the Holocene in Florida

As the concentration of atmospheric carbon dioxide (CO2) as well as global temperatures continue to increase, it is expected that sea level will rise and more extreme precipitation events will occur in the future. Moreover, warmer sea surface temperature may lead to an intensification of tropical storm development. In addition, plants respond to rising CO2 by reducing their transpiration rates, which in turn affects climate and the hydrological cycle. Particularly in densely populated low-lying coastal regions like Florida such changes can have severe economical and ecological consequences. Investigating how landscapes and the hydrological cycle was affected by past changes in climate and sea level allows for a better prediction of future changes. Florida’s estuarine and wetland environments are highly sensitive to small changes in water depth, -chemistry and hydroperiod, and their deposits present excellent archives of past hydrological variability. Multiple palynological, micropaleontological and paleobotanical proxies were applied to describe how changes in sea level and precipitation patterns have affected the landscape during the Holocene, to investigate natural variability in tropical storm activity in this region, and to determine the strength and duration of the plant response to CO2. The current Tampa Bay and Charlotte Harbor estuaries were gradually flooded around 8 ka (kilo y BP) by the rising sea level, changing former freshwater environments to initially lagoonal and later marine environments. During the late Holocene, water salinity increases and a transition from seasonally stratified to a mixed water column is inferred from algal assemblages. Superimposed on this trend, peaking runoff is inferred from increased terrestrial input and stratification indicator dinocyst abundance around 5 ka and 2.5 ka. These runoff phases possibly reflect increased precipitation, related to regionally higher sea surface temperatures (SSTs) and enhanced El Niño-Southern Oscillation activity. However, runoff could also be modified by increased water retaining capacity due to peat and soil development inland, as a consequence of the reduced hydrological gradient as sea level rises. Both trends in precipitation and sea level rise have likely led to the state-wide transition from dry oak to wetter pine dominated vegetation. Lithological, palynological and geochemical evidence from these estuaries suggests tropical storms were regionally more frequent between 6.4-5.5 ka, 5.0-4.0 ka and 3.2-1.9 ka. This variability is likely determined by both increased SSTs and shifts in the position of the Bermuda High. Shifts in vegetation and diatom assemblages during the late Holocene in an elevated central Florida wetland suggests shifts to wetter conditions around ~2.5 and 1.2 ka. These trends are possibly related to regionally warmer SSTs, whereas environmental changes during the 20th century are related to human efforts to protect the site from wildfires. Measurements on leaf fragments from Florida angiosperm, conifer and a fern species, covering the 100 ppm CO2 rise of the past 150 years, show a distinct reduction in maximal leaf conductance of on average 34%. Models predict this plant response will likely continue to beyond double current CO2, which will result in a 50% reduction of canopy transpiration, potentially altering the hydrological cycle and climate

Global CO2 rise leads to reduced maximum stomatal conductance in Florida vegetation

A principle response of C3 plants to increasing concentrations of atmospheric CO2 (CO2) is to reduce transpirational water loss by decreasing stomatal conductance (gs) and simultaneously increase assimilation rates. Via this adaptation, vegetation has the ability to alter hydrology and climate. Therefore, it is important to determine the adaptation of vegetation to the expected anthropogenic rise in CO2. Short-term stomatal opening-closing responses of vegetation to increasing CO2 are described by free-air carbon enrichments growth experiments, and evolutionary adaptations are known from the geological record. However, to date the effects of decadal to centennial CO2 perturbations on stomatal conductance are still largely unknown. Here we reconstruct a 34% (±12%) reduction in maximum stomatal conductance (gsmax) per 100 ppm CO2 increase as a result of the adaptation in stomatal density (D) and pore size at maximal stomatal opening (amax) of nine common species from Florida over the past 150 y. The species-specific g smax values are determined by different evolutionary development, whereby the angiosperms sampled generally have numerous small stomata and high gsmax, and the conifers and fern have few large stomata and lower gsmax. Although angiosperms and conifers use different D and a max adaptation strategies, our data show a coherent response in gsmax to CO2 rise of the past century. Understanding these adaptations of C3 plants to rising CO2 after decadal to centennial environmental changes is essential for quantification of plant physiological forcing at time-scales relevant for global warming, and they are likely to continue until the limits of their phenotypic plasticity are reached

The Amazon at sea: Onset and stages of the Amazon River from a marine record, with special reference to Neogene plant turnover in the drainage basin

The Amazon submarine fan is a large sediment apron situated offshore Pará (Brazil) and represents the most distal extent of the Amazon River. The age of onset of this transcontinental river remains debated, yet is of great importance for understanding biotic evolutionary processes on land and at sea. Here we present new geochemical and palynological data from a borehole drilled at the continental slope and dated based on nannofossil biostratigraphy. We found that sediments of mixed source (craton and adjacent) occur at least from the late Oligocene (NP25) to late Miocene (NN9), and that the earliest Andes-derived sediments occur in NN10 (late Miocene). Our geochemical record indicates an onset of the transcontinental Amazon River between 9.4 and 9 Ma, which postdates the regional unconformity by 1 to 1.5 My. The shift in sediment geochemistry is more gradually replicated in the palynological record by a change from coastal plain and tropical lowland taxa to a mixture of tropical lowland, and montane forest to open Andean taxa. In particular, the appearance of taxa such as Jamesonia and Huperzia, followed by Valeriana, Polylepis-Acaena, Lysipomia and Plantago (with a current altitudinal range from 3200 to 4000 m) suggests the development of open, treeless, vegetation between 9.5 and 5.4 Ma, and highlight the presence of a high Andes in the late Miocene hinterland. Poaceae progressively increased from 9 Ma, with a notable rise from 4 Ma onwards, and percentages well above post-glacial and modern values, particularly between 2.6 and 0.8 Ma. We hypothesize that the rise of the grasses is a basin-wide phenomenon, but that the Plio-Pleistocene expansion of open, treeless vegetation on the Andean slopes and foothills are the main contributor. This rise in grasses was likely caused by climatic fluctuations, and subsequent changes in relief and erosion rates. We conclude that the onset of the Amazon River is coupled with Neogene Andean tectonism and that subsequent developments, both of river and biota, are closely linked to the Plio-Pleistocene climatic fluctuations. From latest Neogene onwards these major landscape changes determined the composition of the montane and lowland forest in the Andes-Amazonian system

Late Holocene sea-level rise in Tampa Bay: Integrated reconstruction using biomarkers, pollen, organic-walled dinoflagellate cysts, and diatoms

A suite of organic geochemical, micropaleontological and palynological proxies was applied to sediments from Southwest Florida, to study the Holocene environmental changes associated with sea-level rise. Sediments were recovered from Hillsborough Bay, part of Tampa Bay, and studied using biomarkers, pollen, organic-walled dinoflagellate cysts and diatoms. Analyses show that the site flooded around 7.5 ka as a consequence of Holocene transgression, progressively turning a fresh/brackish marl-marsh into a shallow, restricted marine environment. Immediately after the marine transgression started, limited water circulation and high amounts of runoff caused stratification of the water column. A shift in dinocysts and diatom assemblages to more marine species, increasing concentrations of marine biomarkers and a shift in the Diol Index indicate increasing salinity between 7.5 ka and the present, which is likely a consequence of progressing sea-level rise. Reconstructed sea surface temperatures for the past 4 kyrs are between 25 and 26 ° C, and indicate stable temperatures during the Late Holocene. A sharp increase in sedimentation rate in the top ∼50 cm of the core is attributed to human impact. The results are in agreement with parallel studies from the area, but this study further refines the environmental reconstructions having the advantage of simultaneously investigating changes in the terrestrial and marine environment. © 2009 Elsevier Ltd