To date our understanding of how hurricane activity has changed over time and how it may change in the future is hampered by the short instrumental and historical record. Sand deposits left behind by hurricanes in coastal sediments provide a means of extending this record. Storm surge and waves associated with hurricanes often deposit sand in salt marshes and coastal ponds. Over hundreds to thousands of years hurricane-induced deposits are laid down and subsequently covered by salt marsh peat creating a barcode-like deposit of sand and mud that provides insight into hurricane frequency. However, we know little regarding the intensity of these past storms. The thickness of the sand deposit and size of sand grains transported during the hurricane has often been used to estimate how intense the hurricane was, but this method is imperfect because sand size and thickness of the hurricane deposits may be largely controlled by the amount of sediment available for transport. We plan to use a small organism called foraminifera that live in all marine environments, to determine where sediment deposited in New England salt marshes by past hurricanes is coming from. These organisms are unique because they live in certain depth ranges offshore, so finding them in past hurricane deposits will help us determine from how deep in the ocean the sand has been excavated. Given more intense hurricanes result in higher waves capable of excavating sediment from greater water depths, deeper water foraminifera assemblages found in hurricane-induced deposits will be indicative of a more intense storm. To test our hypothesis we will use the historic record of hurricane landfalls in southern New England to determine if foraminifera from different water depths, deposited within salt marshes, reflect known hurricane intensities. Once this method is tested we can use it to estimate hurricane intensity from foraminifera in sand layers going back thousands of years. The variation in hurricane intensity over time will provide important baseline information that will help elucidate the climatic controls on hurricane intensity. Results from this work will be useful to coastal resource managers, disaster mitigation managers, and policy makers and will enable these groups to make informed decisions regarding appropriate management practices and regulatory strategies. Other stakeholders (coastal zone managers; habitat restoration groups; land managers; business interests concerned with managing risk; coastal property owners and coastal scientists) will benefit from the results of this project.
Currently efforts to estimate hurricane intensity prior to the instrumental record have been speculative. This project focuses on examining the potential use of foraminifera, a single celled marine organism, that can be found in sediment that is washed into coastal marshes and ponds during hurricanes. Foraminifera live in all marine environments, in the tidal, near- and offshore. This bottom dweller generally lives in abundance in the surface sediment. Different species of foraminifera often occupy different water depth zones (for a number of environmental parameters). Depending on what species are found in sediments associated with known historical hurricane events we have used the depth of modern species for inference. Foraminifera present tidal, near- and offshore environments are used as a way of estimating the depth from where the sediment was excavated during hurricane wave conditions. The assumption being that deeper foraminifera are excavated from greater depths with more intense storms. We have selected a number of key locations which are highly sensitivity to hurricanes along Apalachee Bay, FL in the Gulf of Mexico and southern New England from Connecticut to Massachusetts. We used the archive of historic hurricanes (from ~1600 to present) as a way to validate our findings. At Mullet Pond, FL recent sand layers in the pond sediment were found to be deposited by significant historic storm surges. Using standard sediment dating techniques on a 6m sediment core from the pond bottom, we distinguished 177 events over 4500 yrs. This record indicates that the period with the most hurricanes was from 2800 to 2300 yrs ago and that the frequency over the last 150 yrs is below average. Seven instances in the historic period are coincident with coarse layers in the upper 40 cm of sediment and contain marine foraminifera, not normally found in the pond. Two of these layers had foraminifera which currently reside at depths between -1m and -5m and likely represent Hurricanes Dennis (2005) and the 1941 Hurricane. Three layers had foraminifera from depths exceeding -5m; Elena (1985); 1926 Hurricane and 1894/1896 Hurricanes which are coincident with the most intense events in the historic period. Similarly, at seven coastal salt marshes from Middle Beach CT to Mattapoisett, MA, we reconstructed hurricane records in the top ~60cm of salt marsh peat which covers the historic period. We identified between 4-9 events within the historic period at each site. Coincidently, offshore foraminifera (>-5m depth) were only found in the coarse peaks of the two most severe hurricanes to make landfall in New England during the historic period; the Great Hurricane (1938) and the Great Colonial Hurricane (1635), both category 3 hurricanes at landfall. By looking at each individual hurricanes impact at all the sites together we were able to show that a regional perspective provides value added by shedding light on storm size (number of sites impacted by any given storm) and potentially where storms may have made landfall which we validated with the historic archive. As a valuable indicator of marine sediment source, foraminifera can be used as indicators of past hurricane events. Results using the historic period as a test case to validity their use have proven worthwhile. By using the modern foraminifera-depth relationship they provide some indication of intensity in the form of sediment source depth. When this technique is used in a regional analysis, insight is gained into storm size and potential landfall area. Using this methodology over long records provides valuable storm information not previously attainable which will contribute to the understanding of storm frequency and intensity in conjunction with climate variability.
