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Large and prolonged departures from average precipitation (flood and drought intervals) have significant impacts on the environment and society. Current understanding of the natural cycles of flood and droughts, including changes in frequency and intensity of these cycles associated with climate variability and change, is hampered by the lack of adequate long-term records. With few exceptions, instrumental records extend back less than 100 years. Some historical records can be used to develop longer records of flood and drought, but historical records are limited in number and are often local in nature. For example, logs of U.S. explorers include accounts of drought intervals marked by blowing sand across the Great Plains due to mobilization of sand deposits that are normally stabilized by vegetation. The explorers' accounts of blowing sand indicate that widespread droughts extending from Nebraska to Texas occurred in the early 1820's and 1860's and that several more geographically restricted droughts occurred between 1840 and the early 1860's (Muhs and Holliday, 1995).
Paleoclimate reconstructions derived from tree rings, lake sediments, and continental deposits can be used to extend the record of hydrologic extremes back through time. However, most tree-ring records are restricted to the last 1,000 years. Detailed drought records from lacustrine deposits are limited in number and geographic coverage, and records from continental deposits tend to be fragmented and difficult to date (see Woodhouse and Overpeck, 1998, for a summary). Thus, there is a need for additional long-term records of flood and drought intervals that are continuous and well dated.
The Gulf of Mexico is a semi-enclosed sea that is influenced by processes and conditions on adjacent continental areas (fig. 1). The Mississippi and other major rivers transport freshwater and sediment into the Gulf of Mexico. Several studies indicate that extreme hydrologic events within the Mississippi Basin are recorded in Gulf of Mexico sediments. For example, during the last deglaciation, large amounts of meltwater from the decaying Laurentide ice sheet flowed down the Mississippi River and lowered surface-water salinity throughout the Gulf of Mexico. The reduced surface-water salinity is reflected in anomalously low oxygen-isotope ratios in the shells of surface-dwelling planktic foraminifers that were deposited in Gulf of Mexico sediments during the meltwater event (fig. 2; see discussion in Kennett and others, 1985). The isotopic meltwater signal is strongest in sediments near the mouth of the Mississippi River but is still evident, albeit subdued, in sediments off the coast of Mexico (Kennett and Shackleton, 1975; Leventer and others, 1982).
Analyses of sediments from the Orca Basin (fig. 1) in the Gulf of Mexico provide evidence for several Mississippi Basin flood events during the last 5,000 years. The large flood events, called "megafloods" by Brown and others (1999), are represented in Orca Basin sediments by a combination of changes in the planktic foraminifer assemblages and increases in the grain size of the siliciclastic mud component of the sediments (see discussion in Brown and others, 1999).
Poore and Wright (1999) argued that low-surface-water-salinity events caused by an influx of flood waters into the Gulf of Mexico would result in negative excursions in both lower-case delta13C and lower-case delta18O from the euryhaline surface-dwelling planktic foraminifer Globigerinoides ruber because freshwater has negative lower-case delta13C and lower-case delta18O values relative to seawater. In a reconnaissance study of sediments from the Pigmy Basin (fig. 1) in the Gulf of Mexico, Poore and Wright (1999) identified two negative isotopic excursions in G. ruber isotope data (fig. 3); they indicate that major flood events occurred at about 7,000 and 7,800 radiocarbon years before present.
Provided that discharge variations from the Mississippi River are related to regional-scale climate and major flood events, the identification of post-deglaciation flood events in Orca Basin sediments (Brown and others, 1999) and Pigmy Basin sediments (Poore and Wright, 1999) suggests that a record of regional-scale hydrologic variation for the Great Plains and Central United States extending back for thousands of years is preserved in sediments of the Gulf of Mexico. By analogy with results from studies off of the Mississippi River, we infer that variations in flow from the Rio Grande should be preserved in shelf and slope sediments of the western Gulf of Mexico. Thus, analyses of cores from the western Gulf of Mexico should reveal a record of regional-scale hydrologic extremes in the Southwestern United States.
For example, a number of droughts or closely spaced drought events that were long and severe occurred prior to the 1700's. One or more of these megadroughts affected the Great Plains and Southwestern United States in the second half of the 16th century. Another megadrought occurred during the last part of the 13th century (see Woodhouse and Overpeck, 1998, for summary). Sparse paleoclimate data suggest that four megadroughts affected a wide area of the Great Plains and the Western and Southwestern United States between the 1st and 13th centuries. These megadroughts appear to be century-scale features that were centered around A.D. 300, 775, 850, and 1150 (Woodhouse and Overpeck, 1998). These megadroughts would have clearly reduced flow into the Gulf of Mexico.
Fluvial deposits indicate that the frequency and magnitude of large floods have varied in the upper Mississippi Basin through the Holocene (Knox, 1996). For example, overbank deposits in the upper Mississippi Basin (southwestern Wisconsin, Knox, 1996, fig. 5) suggest that several extreme floods occurred between 5,000 and 6,000 radiocarbon years ago that were larger than any historical flood. In contrast, only minor floods occurred in the upper Mississippi Basin between 5,000 and about 3,000 radiocarbon years ago (Knox, 1996). These extreme floods and the long interval without major floods would have had significant impacts on flow into the Gulf of Mexico.
Our purpose in this study was to determine if variations in discharge from the Mississippi River and Rio Grande reflect major droughts and wet intervals and extreme floods known from historical and proxy terrestrial paleoclimate records. Moreover, since interannual climate variation in the Southwestern United States, the Gulf of Mexico Coast, and the Great Plains is related to the El Niño/Southern Oscillation (ENSO) (Diaz and Kiladis, 1992), we also compared the Mississippi River and Rio Grande records with ENSO indicators to test for an ENSO relation in discharge variations. Documenting a link between known hydrologic extremes and discharge records from the Mississippi River and Rio Grande is part of our long-term effort to develop a Holocene record of flood and drought variability from Gulf of Mexico sediments.
U.S. Geological Survey, U.S. Department of the Interior
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Contact: Harry Dowsett (hdowsett@usgs.gov)
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