Variable response of coastal environments of the northwestern Gulf of Mexico to sea-level rise and climate change: Implications for future change

Abstract

The results from nearly three decades of marine geological research in the northwestern Gulf of Mexico are compiled in an effort to understand those factors (e.g., sea-level rise, sediment supply, subsidence, antecedent topography) that influenced coastal evolution during the last eustatic cycle (~ 120 ka to Present). Armed with this information, we evaluate coastal response to variable sea-level rise of the Holocene and accelerated rise during historical time to gain a better understanding of how the coast is likely to respond to future changes. The early Holocene evolution of northwestern Gulf of Mexico bays was punctuated by rapid and possibly synchronous flooding events that are interpreted as resulting from episodes of rapid sea-level rise. Two of these events, one between ~ 8.4 and 8.0 ka and the other between ~ 7.4 and 6.8 ka, were associated with known episodes of ice sheet retreat in North America and Antarctica, respectively. During the middle and late Holocene, the east Texas and western Louisiana coasts experienced episodes of stability and growth followed by rapid shoreline retreat, while the central Texas coast remained relatively stable. This variability in coastal response to sea-level rise resulted mainly from differences in sediment supply and the highly irregular antecedent topography on which coastal environments formed. Sand that nourished the evolving east and south Texas, as well as westernmost Louisiana, coasts was derived mainly from transgressive ravinement of deltas that were formed during the falling stage of sea level (MIS5e–MIS2). The loci of these deltas controlled the spatial variability of this offshore sand supply. Sand supply to the central Texas coast has been dominated by converging longshore currents and throughout the middle to late Holocene was large enough to keep pace with sea-level rise. Moreover, sand supply from rivers has varied considerably in response to climate change. During the early Holocene, when the average rate of linear sea-level rise was 4.2 mm/yr, the Brazos, Colorado and Rio Grande Rivers all formed deltas on the inner continental shelf. Today, only the Brazos contributes enough sediment to the Gulf of Mexico to form a delta, but its sand delivery is minimal. A reversal from late Holocene growth of western Louisiana–east Texas and south Texas barrier islands and peninsulas to erosion occurred during the last ~ 2.0 ka, after the rate of sea-level rise began to decelerate. The actual timing of this reversal was diachronous across the coast and was caused by a decrease in sediment supply once offshore sand sources were bypassed by the landward-advancing shoreface (transgressive ravinement). Differences in the exact timing and rate of retreat also reflect variability in the offshore profiles of these barriers and their thickness, which results from the underlying relief on the Pleistocene surface. The late Holocene was also the time of chenier plain development in western Louisiana and east Texas that was marked by localized differences in growth and retreat. The current rate of eustatic sea-level rise in the northern Gulf of Mexico is in the range of 2.0 and 3.0 mm/yr and is approaching rates of the early Holocene. This is on average about five times the long-term rate of the past 4.0 ka. This larger rate is associated with an increase in coastal erosion in historical time relative to long-term rates. Current rates of Gulf shoreline erosion are approaching rates of the early to mid-Holocene and the rate of retreat of some barriers is not sustainable. Core transects off central Texas reflect a late Holocene history of shoreface progradation, but the modern shoreline is currently eroding, albeit at slower rates than elsewhere along the coast. Overall, the sand supply needed to sustain the Gulf shoreline is lacking. Likewise, baylines are experiencing erosion rates that are faster than the late Holocene average and bayhead deltas are at the tipping point of catastrophic retreat. Spatial variability in erosion rates highlights the importance of factors such as sediment supply, subsidence, and anthropogenic influences on coastal evolution. Tropical storms and hurricanes, while exerting the most noticeable coastal change, merely exacerbate overall shoreline retreat and migration. Along the entire northwestern Gulf Coast, the impact of human intervention to natural processes is particularly evident, primarily due to alterations in river discharge and sediment supply to the coast, interruptions in along-shore and cross-shore sand transport, and increases in rates of subsidence through fluid withdrawal. Compared to natural changes that took place during the Holocene, the impacts of human activity have been virtually instantaneous. The geological record indicates that the ongoing increase in the rate of sea-level rise, coupled with diminished sediment supply and human intervention, will continue to severely impact the low-gradient coastal environments of the northwestern Gulf of Mexico.