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M. Marot

Publications and source records attributed to M. Marot.

6 recordsLinked to original sources
USGS2007

Synthesis of age data and chronology for Florida Bay and Biscayne Bay cores collected for ecosystem history of South Florida’s estuaries project

210 Pb, 14 C, and pollen biostratigraphic data have been compiled and synthesized to develop age models for cores collected from Florida Bay and Biscayne Bay. These cores are being used to interpret the ecosystem history of south Florida’s estuaries by examining the physical, chemical, and biological record preserved within the cores. The beginning of the 20th century, which marks an important turning point for the natural vs. anthropogenically influenced ecosystem, has been identified based on at least two data points in ten cores. 210 Pb data alone are presented for an additional 38 cores. Age models for older sediments have been developed for seven cores. Comparison of pre-1900 and post-1900 records allows researchers to compare natural ecosystem changes to anthropogenic change. General patterns of sedimentation rates in Florida Bay and Biscayne Bay emerge from the data. Mid-bay mudbanks in both bays show more rapid rates of sedimentation, fewer signs of sediment disruption, and more internal consistency of sediments than cores located closer to shore. Nearshore cores indicate slower average rates of sedimentation, more disruption in the sedimentary sequences, and more indications of “old” carbon effects. Cores in close proximity to each other generally show very similar patterns of deposition, which indicates support for the age models.

Southern Florida↗
USGS2004

Anthropogenic and natural variation in ridge and slough pollen assemblages

We present pollen evidence documenting the response of sawgrass ridge and slough systems of the Florida Everglades to hydrologic changes during the last 3,500 years. Sediment cores and surface samples were collected in three transects across sawgrass ridges and sloughs in Water Conservation Area 3A to determine the age of the features, long-term variability in plant community composition, stability of sawgrass ridge and slough size, and their response to 20th century changes in hydrology. Statistically significant differences in abundance of Cladium pollen in surface samples collected throughout the system allow differentiation of these communities in the sedimentary pollen record. Analysis of pollen in cores from the three transects indicates that the general distribution of ridges and sloughs has remained distinct through time. There is evidence the vegetation has responded to past global-scale climate events, such as the Medieval Warm Period, as well as the 20th century anthropogenic alterations to the natural hydrology. The ridge community is more responsive and susceptible to perturbations in hydrology than the slough community. In contrast, the slough plant community is more stable and less likely to demonstrate long-term changes after perturbations to hydrology. Regardless, these data indicate that the ridge and slough landscape is resilient to changes in hydrology and posses the potential to return to a natural state with the return of natural hydrologic conditions.

Florida↗
USGS2002

Accumulation rate and mixing of shelf sediments in the Monterey Bay National Marine Sanctuary

The distribution of excess 210Pb in 31 sediment cores was used to determine modern (last 100 yr) mass accumulation rates and the depth of sediment mixing on the continental shelf between Pacifica and Monterey, California, USA. Apparent mass accumulation rates average 0.27 g cm-2 yr-1 and range from 0.42 g cm-2 yr-1 to 0.12 g cm-2 yr-1. Accumulation rates were highest at mid-shelf water depths (60-100 m) adjacent to major rivers and near the head of the Ascension submarine canyon. Cores from water depths of less than 65 m had low, uniform 210Pb activity profiles and sandy textures. The uppermost 5-13 cm of 15 cores had uniform 210Pb activity profiles above a region of steadily decreasing 210Pb activity. This phenomenon was attributed to sediment mixing. The thickness of this upper layer of uniform 210Pb activity decreased southward from 13 cm, west of Pacifica, to less than 5 cm, near Monterey Canyon. This southward decrease may be attributed to shallower bioturbation in the southern study area. Integrated excess 210Pb activities were generally higher where sedimentation rates were high. They were also higher with increasing distance from major rivers. Thus, sedimentation rate alone does not explain the distribution of integrated excess 210Pb in this study area. Excess 210Pb in the seafloor is controlled by other factors such as sediment texture, the atmospheric deposition rate of 210Pb, and the residence time of sediment particles in the water column. ?? 2002 Elsevier Science B.V. All rights reserved.

Marine Geology↗
USGS2000

Historical trends in Chesapeake Bay dissolved oxygen based on benthic foraminifera from sediment cores

Environmentally sensitive benthic foraminifera (protists) from Chesapeake Bay were used as bioindicators to estimate the timing and degree of changes in dissolved oxygen (DO) over the past five centuries. Living foraminifers from 19 surface samples and fossil assemblages from 11 sediment cores dated by 210 Pb, 137 Cs, 14 C, and pollen stratigraphy were analyzed from the tidal portions of the Patuxent, Potomac, and Choptank Rivers and the main channel of the Chesapeake Bay. Ammonia parkinsoniana , a facultative anaerobe tolerant of periodic anoxic conditions, comprises an average of 74% of modern Chesapeake foraminiferal assemblages (DO-0.47 and 1.72 ml l −1 ) compared to 0% to 15% of assemblages collected in the 1960s. Paleoecological analyses show that A. parkinsoniana was absent prior to the late 17th century, increased to 10–25% relative frequency between approximately 1670–1720 and 1810–1900, and became the dominant (60–90%) benthic formaniferal species in channel environments beginning in the early 1970s. Since the 1970s, deformed tests of A. parkinsoniana occur in all cores (10–20% of Ammonia ), suggesting unprecedented stressful benthic conditions. These cores indicate that prior to the late 17th century, there was limited oxygen depletion. During the past 200 years, decadal scale variability in oxygen depletion has occurred, as dysoxic (DO=0.1–1.0 ml l −1 ), perhaps short-term anoxic (DO<0.1 ml l −1 ) conditions developed. The most extensive (spatially and temporally) anoxic conditions were reached during the 1970s. Over decadal timescales, DO variability seems to be linked closely to climatological factors influencing river discharge; the unprecedented anoxia since the early 1970s is attributed mainly to high freshwater flow and to an increase in nutrient concentrations from the watershed.

Maryland, Virginia↗
USGS1999

Geochemistry of Florida Bay sediments: Nutrient history at five sites in eastern and central Florida Bay

Recent seagrass dieoff and massive microalgal blooms have focused attention on the health of the Florida Bay ecosystem. Changes in nutrient input and the nutrient dynamics of Florida Bay are hypothesized to be linked to these problems, but crucial baseline information is still lacking. Efforts to restore Florida Bay to its natural condition will require information on the nutrient history of the bay. The purpose of this study was to examine distributions of organic C, total N, and total P in carbonate sediments from sites of continuous and known sedimentation rate (210Pb and 137Cs dated), in eastern and central Florida Bay. These sediments provide a record of historical changes in the C, N, and P load to the eastern and central bay. Analyses were conducted on sediments from cores collected at five sites, and on buried seagrass fragments at two sites. At three of the sites, sediments from seagrass-covered and adjacent barren areas were examined to determine differences in sedimentary geochemistry. Stable isotope analyses (??13C and ??15N) of sedimentary organic C and total N and of buried seagrass fragments were also carried out at two sites to examine possible changes in nutrient sources to the estuary. Results were consistent with recent increases in N and P in eastern Florida Bay, beginning in the early to mid 1980's. The timing of the increase in nutrient load observed in the sediment data directly preceded the first observations of massive microalgal blooms and seagrass dieoff in Florida Bay in 1987. The observed nutrification was greater for P than N, and was most pronounced at the most northeasterly site sampled (Pass Key). Isotope data (??15N) suggested that an increase in algal production accompanied the increase in N load at the Pass Key site. Along record of organic C, total N, and total P distributions from Whipray Basin in central Florida Bay showed historical peaks (mid 1700's and late 1800's) in organic C and total N, but not total P; these enrichments were nearly equivalent to recent inputs to the estuary. Barren areas were observed to have generally lower concentrations of organic C, total N, and total P in near surface sediments compared to seagrass-covered areas, but had generally similar concentrations in deeper sediments. This suggested that barren areas adjacent to seagrass-covered sites were places where relict sediment was physically transported and covered seagrass beds. This dataset provides an historical view of changes in nutrient inputs to Florida Bay, and baseline information needed for nutrient modeling of the bay.

Journal of Coastal Research↗