Evolution of the Mississippi Floodplains During the Holocene
By Peter Clift
Changes in climate and environment are known to have impacted the development of landscape and have sometimes been linked to the tectonic evolution of our planet, such as the proposed links between the growth of the Himalayas and the onset of the Asian monsoon. Change in rainfall patterns and strength will influence patterns and rates of erosion. However, it is not only climate change that can influence erosion of the landscape, but also human activities most notably agriculture which disrupts soils, especially on hillslopes, leading to rapid increases of sediment flux to the ocean. This can cause problems for geologists who often use modern river composition as fingerprints for looking at ancient sedimentary systems. For example, the mineral content of the Mississippi today can be used to understand its influence on the sediment accumulating in the Gulf of Mexico in the past. However, this simple application is more complicated if the modern river has been disrupted by human activities resulting in the sediment load not being representative of its compositions in the geological past. The impact of humans on drainage basins has been documented in Africa
and more recently in southern China where the onset of advanced agriculture has resulted in substantial changes to the chemistry and mineralogy of those rivers. We are now interested in understanding how the Mississippi has evolved in this respect. It might be expected that the European settlement of the Great Plains, largely during the 19th century, would be the largest single influence on the river that we see today, but this hypothesis has yet to be put to the test. In a new study my student Brittney Gregory and I are examining core taken from across the Mississippi Delta to examine the evidence for changes in composition over the last 10,000 years. We are also looking at the composition of the major tributaries that contribute sediment into the mainstream of the Mississippi that are seen and can be sampled in southern Louisiana. By fingerprinting the sediments from the different sub-basins we hope to be able to see differences in the composition of the Mississippi system through time. Our goal is to be able to reconstruct changes in the source of sediment reaching the delta in the recent geologic past. For example, the start of widespread agriculture on the Great Plains might be
expected to increase sediment flow in those rivers draining that area (e.g., Missouri and Arkansas Rivers) compared to those bringing material from the Appalachian Mountains (e.g., the Ohio River). Disruption of the Mississippi may be much older than the onset of European settlement because of humans reaching the continent from Asia over the Bering Strait during the last glacial period, which later caused deforestation and large-scale landscape impact, as evidenced by the occurrence of large-scale burial mounds dating to around 1,000 years ago seen across the delta and lower reaches. This provides us with evidence of significant population density and of large-scale landscape disruption greatly predating European settlement. Our aim is to understand the influence that humans have had on the Mississippi system over thousands of years both to appreciate the scale anthropogenic impacts on the basin, and to clarify what we can really learn about natural erosion processes by looking at the
Holocene delta. By having a greater understanding of the compositional range of the Mississippi, we improve our abilities to interpret long duration geological sedimentary records from the offshore Mississippi submarine fan since its onset. Our research involves dating of zircon crystals that are transported as sand grains and whose range of crystallization ages provide us with unique signatures that allow us to trace their origins back to source bedrocks in the headwaters. We will further be looking at the types of heavy minerals in these sands, such as pyroxenes, amphibolies, and iron oxides, as well as clay minerals to see whether there are distinctive fingerprints from the different source regions and to see how they evolve through time. We aim to get a better understanding about how sediment is transported from source into the deep-water gulf and what influences this flux.
Improving our understanding of sediment production and transport in the largest drainage basin in North America will have profound implications for our understanding of the landmass, as well as with the interpretation of ancient river deposits both in North America and worldwide. This work is supported by funding from the Charles T. McCord Jr., chair in petroleum geology.