Research articleDeltaic response to climate change: The Holocene history of the Nueces Delta
Introduction
Coastlines are dynamic environments changing at timescales from minutes to millennia. As important transitional zones between the marine and terrestrial realms, coasts are influenced by a number of important factors and processes, including sea-level changes (Fitzgerald et al., 2008; Rodriguez et al., 2010), subsidence (Törnqvist et al., 2008), tectonics (Simms et al., 2016), and climate-driven changes in sediment supply (Livsey and Simms, 2016), as well as internal feedbacks from physical processes operating within the coastlines themselves (Muto and Steel, 1992; Simms and Rodriguez, 2014; Ciarletta et al., 2019). Their increasingly urban nature and rapid development have led researchers to try and predict or better understand how coastlines might respond to future changes in the rate of sea-level rise. Numerical models have been developed to predict coastline behavior (Ciarletta et al., 2019) and changes documented throughout the Holocene and late Pleistocene have been used as analogues for what might be expected in the future (Anderson et al., 2014). The advantage of using the Holocene over older time periods is the higher preservation of these recently deposited sedimentary archives and the wide availability of methods for determining the age of the deposits, such as radiocarbon dating and optically stimulated luminescence. In addition, we arguably understand more about the last 10,000 years than any other time period in Earth's history; thus, independent records of several allogenic (e.g., tectonic activity, sea-level changes, climatic changes, etc.) forcings are available. Of these forcing mechanisms, past changes in the rate of sea-level rise are often the most studied (e.g., Fitzgerald et al., 2008). However, other processes that influence coastal change have changed in the past and will change in the future. One of these important processes that has received little attention is climatically-controlled changes in sediment supply (Zhang et al., 2019).
The purpose of this study is to examine the Holocene history of the Nueces Delta to determine how the delta behaved during periods of independently documented climatic changes in the middle to late Holocene. This evolution is deduced using the stratigraphic changes recorded in sediment cores and high-resolution seismic profiles. Specifically, the location of the mouth bar that forms at the front of the bayhead delta was mapped over the last 8.5 ka. Additionally, radiocarbon ages and grain-size data are used to quantify the sand delivered to the delta over the past 5 ka, which is used as a proxy for changes in the sediment flux to the delta. This calculation was conducted in order to help ascertain whether the changes were likely due to simply autogenic processes operating within the delta or controlled by climate. If climate was the driver of these changes, it most likely did so through modulating the sediment flux delivered to the delta.
Section snippets
Background
The Nueces River of the northwestern Gulf of Mexico empties into upper Nueces Bay, a tributary bay of the larger Corpus Christi Bay, and forms the Nueces Bayhead Delta (Fig. 1). The bayhead delta hosts the Port of Corpus Christi, Texas, the 3rd largest port by tonnage in the United States (https://portofcc.com/about/port/about-us/; last accessed August 2019). The bay lies along a low gradient coast with an average slope of 0.28 m/km (Morton and McGowen, 1980). It is located in a sub-humid to
Methods
We obtained 38 cores up to 15 m in length and 25 km of high-resolution seismic data from the Nueces delta (Fig. 2). Eight of the cores were obtained using a 5.1 cm diameter GeoProbe while the remaining 30 cores were obtained using a 7.2 cm diameter vibracore. Cores were cut, photographed, and described at the University of California Santa Barbara sedimentology lab. Forty-five new AMS radiocarbon ages were obtained from shell material or, in one case, a wood fragment from the cores (Table 1).
Sedimentary facies and interpretations
Ten vibracores were obtained in and around the active lobe of the Nueces Delta to provide a modern analogue when interpreting older facies within the longer cores. Within these cores, 6 different facies were identified (Table 2; Fig. 3). Core ND12-33V contained all 6 of these sedimentary facies within a framework that aided their interpretation. Core ND12-33V was taken within an abandoned distributary channel of the Nueces Delta and landward of the modern mouth bar (Fig. 3). The upper 38 cm of
Delta lobes
The mouth bar represents the seaward extent of the delta shoreline and its position was used to track the evolution of the delta through the Holocene similar to the approach of Chamberlain et al. (2018). Six new mouth-bar complexes representing 6 delta lobes, not including the modern, active delta lobe, were identified in cores and seismic profiles within this study (Lobes 2, 4, 5, 6a, 6b, and 7 on Fig. 2). Of the six new lobes identified primarily within cores, four (Lobes 5, 6a, 6b, and 7)
Conclusions and implications
Our data show that natural coastal systems are very dynamic and can retreat and prograde at rates exceeding 10 m/yr in the absence of global fluctuations in sea level. Following transgression during the early Holocene, which was likely driven by high rates of relative sea-level rise, the Nueces delta retreated during periods of decreasing sea levels through the late Holocene. Thus a mechanism other than increasing rates of RSL rise is responsible for these latter transgressive events at around
Declaration of Competing Interest
We declare no conflict of interest.
Acknowledgements
We would like to thank Jake Herring and the Coastal Bend Bays and Estuaries Program for allowing us to work on the delta. We thank John Anderson for introducing us to working in the region and for encouraging this study. Baird King and Brooke Briand helped with fieldwork. This manuscript was also improved by thoughtful reviews by two anonamous reviewers. This project was supported by NSF Grant EAR-0921963 and student grants through GSSEPM and AAPG.
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- 1
Current affiliation: DCOR LLC, Ventura, California, USA.
- 2
Current affiliation: Queens University, Kingston, ON, Canada.
- 3
Current affiliation: USGS, Sacramento, CA, USA.
- 4
Current affiliation: Rutgers University, New Brunswick, NJ, USA.