Holocene sediment accretion in the Trinity River delta, Texas, in relation to modern fluvial input

Abstract

Purpose

This study uses sediment cores to quantify Holocene sedimentation rates in the Trinity River delta, Texas. An important question is whether modern fluvial sediment input from the Trinity River is adequate to sustain sedimentation in the delta, thereby combating subsidence and further wetland loss. Our objective was to quantify sedimentation rates within the delta in order to assess whether the delta is in- or out-of-phase with modern sediment delivery rates.

Materials and methods

Two core samples were retrieved from the subaerial Trinity River delta, an intricate system of anastomosing/bifurcating channels, levees, lakes, and interdistributary marshes covering an area of 126.1 km². Detailed descriptions of sample characteristics were recorded, including wet and dry color, texture, moisture content, and plasticity. Bulk density and grain-size measurements were also conducted on the cores. Shell fragments from each sediment core were subjected to accelerator mass spectrometry ¹⁴C dating.

Results and discussion

Radiocarbon dating of shell fragments recovered from sediment cores (292–536 cm) ranged from 2.8 to 3.2 Ka. Mean sediment accumulation rates varied between 1.2 and 1.8 mm year^–1 for two areas of the delta, consistent with previous research. The amount of sediment needed to cover the surface area of the modern Trinity River delta was calculated to obtain the approximate amount necessary for accretion rates from 0.25 to 3.0 mm year^–1. Using the mean Holocene sediment accretion rate from this study (1.5 mm year^–1) to cover 100% of the Trinity River delta, the amount of sediment needed per year was calculated to be 265,000 Mg year^–1, which is 3.8 times the amount of sediment (i.e., 70,000 Mg year^–1) currently delivered to the delta by the Trinity River. Modern fluvial sediment input from the Trinity River is thus inadequate to account for Holocene sedimentation rates in the delta.

Conclusions

Sediment accumulation rates of 1.2 and 1.8 mm year^–1 were attained for two interdistributary marshes in the Trinity River delta by radiocarbon dating of shell fragments recovered from sediment cores. These accumulation rates are well within the ranges established in previous studies. However, modern fluvial sediment input from the Trinity River to the delta is occurring at a rate of approximately 0.4 mm year^–1, which is insufficient to combat subsidence and wetland loss and keep pace with future sea level rise.