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St. Petersburg Coastal and Marine Science Center

Climate and environmental change in the Gulf of Mexico and Caribbean

Research: Late Holocene Paleoclimate and Hypoxia of Coastal Sediments

Late Holocene Paleoclimate

Coastal sediments can serve to link climate proxy records from terrestrial and marine systems. This project will primarily use benthic foraminifera and incorporate other proxies (pollen, organic and inorganic geochemical) in marine and marsh sediment cores to identify anthropogenic change and natural variability including climatic cycles, salinity, and sea level records on various time scales. Current work focuses on the late Holocene (to 6,000 BP) climate/hurricane record of Mobile Bay and climate/sea level records for southern Alabama and Louisiana marsh cores. This research is done in collaboration with other USGS scientists.

Map showing location of three Mobile Bay box cores    Changes in the percent agglutinated foraminifers in three dated box cores from Mobile Bay, Alabama.
Figure 1. (Above left) Map showing location of three Mobile Bay box cores seen in Figure 2. [larger version]

Figure 2. (Above right) Changes in the percent agglutinated foraminifers in three dated box cores from Mobile Bay, Alabama. Faunal and geochemical analyses show that dredging and spoil disposal caused reduced circulation resulting in the replacement of calcareous with agglutinated foraminiferal specimens. Prior to construction, beginning in 1880, agglutinated foraminifers were a minor component of the assemblage. Currently they are the dominant form throughout the bay (Osterman and Smith, 2012). [larger version]

Foraminifers and sediments document that over the last century channel dredging and spoil disposal have altered circulation, reduced estuarine mixing, changed sedimentation patterns, and caused a faunal turnover within Mobile Bay. Since the completion of the current shipping channel in the 1950s has further restricted tidal flushing and increased terrestrial organic matter accumulation in the bay. In deeper areas of the bay, hypoxic water has negatively impacted the marine microfauna. Comparisons of the present-day microfossil assemblage with those from the 1970s indicate that the continued biologic loss of calcareous foraminifers in the bay has allowed the introduction of a new foraminiferal species into the bay.

Calcareous foraminifers collected from modern Mobile Bay sediments show test (shell) dissolution and reveal recently living protoplasm (pink stain) and organic inner linings (brown)    Figure 3. (Left) Calcareous foraminifers collected from modern Mobile Bay sediments show test (shell) dissolution and reveal recently living protoplasm (pink stain) and organic inner linings (brown). Ship channel dredging and spoil disposal have reduced bay circulation and resulted in increased dissolution over the last 100 years. Photo Credit: Lisa Osterman. [larger version]

Paleo-Hypoxia

In this project we have documented that a specific faunal proxy, termed the PEB index (= % Protononion atlanticum + % Epistominella vitrea + % Buliminella morgani), is statistically representative of the modern seasonal Louisiana hypoxia zone. The PEB index can be used as a low-oxygen proxy to detect past hypoxic (dissolved oxygen <2 mg L-1) conditions in the Holocene-aged sediments from the Louisiana and Texas continental shelves. Records of past changes in bottom water oxygen content allow an understanding of the roles of natural variability versus human activities in the development of seasonal hypoxic conditions in the Gulf of Mexico.

Our research demonstrates that the development of low-oxygen bottom water on the Louisiana shelf is a natural process that has occurred periodically for at least the last 1,000 years in response to natural climate variations. However hypoxia became a chronic problem about 50 years ago as anthropogenic influences increased in the Mississippi Basin (Fig. 5). Our findings suggest that future population growth, and land use/ land cover changes combined with climate variability and potential climate change may cause chronic hypoxia to spread to other regions of the northern Gulf Coast.

map showing PEB hypoxia index.    map showing Increasing percent PEB in dated sediment cores from the Louisiana shelf hypoxic zone
Figure 4. (Above left) Gulf of Mexico seafloor bottom sediments that experience seasonal hypoxia contain higher relative percentages of three benthic foraminiferal species. The cumulative percentage of these species forms the PEB hypoxia index, which is used to identify past occurrences of seasonal hypoxic conditions. [larger version]

Figure 5. (Above right) Increasing % PEB in dated sediment cores from the Louisiana shelf hypoxic zone allow us to map the expansion of anthropogenically caused hypoxia during the 20 C. [larger version]

Publications

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