The evolution of a subaqueous delta in the Anthropocene: A stratigraphic investigation of the Brazos River delta, TX USA

Continental Shelf Research 111 (2015) 139–149

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The evolution of a subaqueous delta in the Anthropocene: A stratigraphic investigation of the Brazos River delta, TX USA Joseph A. Carlin a,n, Timothy M. Dellapenna b,c a

Department of Geological Sciences, California State University, Fullerton, Fullerton, CA 92834, United States Department of Marine Sciences, Texas A&M University – Galveston, Galveston, TX 77553, United States c Department of Oceanography, Texas A&M University, College Station, TX 77843, United States b

art ic l e i nf o

a b s t r a c t

Article history: Received 1 April 2015 Received in revised form 8 July 2015 Accepted 7 August 2015 Available online 19 August 2015

Globally, deltas are increasingly threatened by anthropogenic activities. As a result, deltas now evolve through the combined effects of natural and human-induced processes occurring throughout the fluvial– deltaic system. The Brazos River delta, located along the Texas coast in the northwestern Gulf of Mexico, and its watershed have been impacted by direct and indirect human activities since the late 19th century. This provides an opportunity to investigate how such alterations have shaped the evolution of a delta in the Anthropocene, a time when humans are drivers of geological change. Historic alteration to the delta and watershed include extensive agricultural activity, jetty construction at the mouth in the late 1890s, mouth diversion
10 km to the southwest in 1929, and reservoir construction throughout the early and mid 20th Century. Three subaerial deltaic geometries provided the framework to connect subaerial deltaic responses, to the anthropogenic alterations, to the resulting stratigraphic characteristics observed in the subaqueous delta. This study utilized high-resolution geophysical data (swath bathymetry, side scan sonar, CHIRP subbottom profiling) on the subaqueous delta to investigate the subaqueous delta stratigraphy and infer the processes that shaped the deltaic record over time. The results showed distinct areas across the subaqueous delta that were dominated by erosion and deposition. Erosional areas corresponded to earlier growth phase depocenters being exposed at the surface, while the depositional areas corresponded to areas with the most recent growth phase depocenter overlying the earlier depocenters. These results highlight that the subaqueous depocenter has migrated westward over time, consistent with the observed changes to the subaerial delta. Additionally, the data showed that evidence for these past growth phases and depocenters may be preserved within the subaqueous delta, even after subaerial portions of the delta returned to pre-Anthropocene configurations. In general, this study showed that the subaqueous delta, similar to the subaerial delta, is impacted by human-driven alterations to the fluvial–deltaic system, and may preserve a more complete record of the evolution of deltas during the Anthropocene. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Delta evolution Human-induced change Clinoform abandonment Shoreline progradation Brazos River Texas coast

1. Introduction The physical, chemical, and biological systems of global deltaic environments are experiencing observable anthropogenic impacts from the nearly 500 million people living on or near them (Baumann et al., 1984; Coleman and Wright, 1975; Correggiari et al., 2005; Day et al., 2000; Longhitano and Colella, 2007; Neill and Allison, 2005; Simeoni and Corbau, 2009; Syvitski, 2008; Syvitski and Kettner, 2011; Syvitski et al., 2009; Syvitski and Saito, 2007). Human impacts throughout Earth's environments have caused some to propose we have entered a new geological epoch, the n

Corresponding author. E-mail address: [email protected] (J.A. Carlin).

http://dx.doi.org/10.1016/j.csr.2015.08.008 0278-4343/& 2015 Elsevier Ltd. All rights reserved.

Anthropocene, a time where many of the planet's key processes are significantly influenced by humans (Crutzen, 2002; Zalasiewicz et al., 2011). Specifically, these fluvial–deltaic systems are increasingly being shaped by human forces that control the flow of water and material throughout the system (Syvitski and Saito, 2007). This human imprint on coastal environments has been under-appreciated (Syvitski and Kettner, 2011), and understanding deltaic responses to the interplay of natural and anthropogenic forces will be critical to both the environment and human populations in the future. Delta morphology is the summation of fluvial characteristics (e.g., river length, maximum discharge, and sediment load) and the relative dominance between sediment remobilization due to marine processes (waves and tides) and fluvial sediment supply at the mouth (Bhattacharya, 2006; Bhattacharya and Giosan, 2003;

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Coleman, 1982; Coleman and Wright, 1975; Galloway, 1975; Geleynse et al., 2011; Nittrouer et al., 1986; Scruton, 1960; Syvitski and Saito, 2007; Walsh et al., 2004; Wright, 1985; Wright and Coleman, 1972, 1973). Human activities can alter any of these variables, thereby altering delta morphology, with the record of these changes being preserved within the delta stratigraphy. For example, land use changes in the watershed can alter riverine sediment load and natural distributary-channel abundance, in turn changing local water retention and surficial flow paths (Milliman and Syvitski, 1992; Syvitski and Kettner, 2011). The Po River system typifies such impacts. Deforestation in the watershed increased sediment flux to the delta (Trincardi et al., 2004) leading to a river mouth diversion in the late 16th to early 17th centuries to prevent sediment from closing the lagoon mouths in the Veneto region ultimately creating a new delta (Simeoni and Corbau, 2009). From the Po River, we see that human intervention can dramatically change a delta to a point where an Anthropocene delta is created as human activities disrupt the natural tendencies of the system. As human populations and their impacts on fluvial–deltaic systems increase, it is important to understand the resulting impacts on the deltaic environments, and the evolution of the new Anthropocene deltas. The purpose of this study is to investigate an Anthropocene delta, which we define as a delta that has formed or evolved under the interplay between natural and human forces. The Brazos delta is an ideal study site as human intervention to this system is

relatively recent (o200 years), and mostly occurred during a period (1929–present) when instrumented hydrographic observations were recorded and available. Previous work on the Brazos delta has investigated shoreline and bathymetric changes over time (Carlin and Dellapenna, 2014; Rodriguez et al., 2000; Seelig and Sorensen, 1973), the evolution of the facies architecture of the subaerial and subaqueous deltas (Fraticelli, 2006; Rodriguez et al., 2000) and sedimentation on the subaqueous delta (Carlin and Dellapenna, 2014; Rodriguez et al., 2000). This study aims to build on this previous research through a high-resolution geophysical survey of the subaqueous delta. The subaqueous portion of deltas reflect the balance between fluvial sediment supply and regional oceanographic conditions (Kuehl et al., 1997; Nittrouer et al., 1996; Palinkas and Nittrouer, 2007; Walsh et al., 2004) and through investigating the stratigraphic characteristics of the subaqueous delta we aim to achieve a better understanding of how the subaqueous delta has responded to anthropogenic alterations over time, and how these changes may be preserved within the geologic record.

2. Background The Brazos River is the 11th longest in the United States and empties directly into the Gulf of Mexico (GOM), forming a wavedominated delta (Fig. 1) that is largely subaqueous (Rodriguez et al., 2000). The delta is situated on a wide, micro-tidal shelf

Fig. 1. Anthropocene growth phases of the Brazos River delta based on past shoreline configurations (past shoreline configuration data downloaded from the NOAA Shoreline Data Explorer: http://www.ngs.noaa.gov/RSD/shoredata/NGS_Shoreline_Products.htm). (a) Illustrates the relative location and timing of the three growth phases: the Old Mouth growth phase (red), New Mouth growth phase (blue), and Western Flank growth phase (green) overlaid on the 2013 coastal configuration (gray). Characteristic shoreline configurations for each growth phase are shown in (b) for the Old Mouth growth phase with the 1933 shoreline compared to 2013, (c) the New Mouth growth phase with the 1948 shoreline compared to 2013, and (d) the Western Flank growth phase with the 2013 shoreline. In all three panels the dashed line represents the 1852 (pre-Anthropocene) shoreline configuration, and the color shading correspond to the earlier growth phases. In (e) the changes in shoreline position are represented graphically for each of the growth phases. Shoreline change is relative to the 1852 position where positive numbers represent a seaward advance of the shoreline and negative numbers are a landward retreat of the shoreline (the black, dashed, horizontal line represents the 0 position). The color shading in the background of the plot corresponds to the specific growth phase at that given time. The locations of the measurements are shown by the dashed, shore-normal lines in (a). The same color scheme for each growth phase will be used in subsequent figures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. Brazos River water discharge and sediment load data. (a) Average annual water discharge (blue circles) with 5 year moving average (blue line), and average annual sediment load (filled brown squares) with 5 year moving average (solid brown line) for the Brazos River. Data is from the gage station at Richmond, TX (location shown in b) courtesy of the USGS and Texas Board of Water Engineers. Suspended load data for the river was not collected after 1986; those values (open brown squares, and dashed brown line 5-year moving average) were estimated from a rating curve (after Syvitski et al., 1987) developed from the relationship between average annual discharge and average annual sediment load data (r2 ¼ 0.81) from the Richmond, TX gage station for the years 1957–1986. Background colors correspond to dominant growth phases from Fig. 1. (b) Map of the Brazos River (solid black line) and watershed (shaded area). The location of the Richmond, TX gage station (red square) is shown relative to the delta (yellow star). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(
130 km), with 1.1 m average wave heights (McGowen et al., 1977). Net longshore drift is dominantly east to west for most of the year (Curray, 1960; Nowlin et al., 2005; Seelig and Sorensen, 1973). Human alterations to the system likely began in the 19th century when the first American settlement on the Brazos River was established in 1821 (Ledbetter, 1959). By the latter half of the 19th century, much of the river's middle and lower drainage basin was converted from open prairie and hardwood forest to agriculture and grazing land (Dunn and Raines, 2001; Ledbetter, 1959). In the late 1890s and early 20th century direct anthropogenic alterations to the river mouth resulted in rapid and dramatic changes to the delta. These changes have been discussed previously by Seelig and Sorensen (1973) and Rodriguez et al. (2000) and are summarized below. The first significant alteration to the delta occurred when jetties were constructed at the mouth in the late 1890s. These jetties caused the delta to prograde
1.5 km seaward, and develop a strong asymmetry to the west, illustrated by the 1933 shoreline (red area) in Fig. 1b. The jetties interrupted longshore transport, which allowed sediment to rapidly accumulate down drift (Seelig and Sorensen, 1973). The second, and most significant alteration was the diversion of the mouth in 1929 from its natural course to its present location, approximately 10 km to the southwest (“Old Mouth” and “New Mouth” in Fig. 1a). The diversion of the river mouth creates a distinction between the Old Brazos Delta associated with the natural mouth, and the New Brazos delta that formed following the diversion. When the mouth was diverted in 1929, the New Brazos delta began forming immediately. While this new delta was rapidly prograding seaward, the Old Brazos delta was eroding, and retrograded at approximately the same rate. By 1948 (blue area, Fig. 1c), the subaerial portion of the new delta had reached its most seaward position, developed a cuspate shape with the river mouth deflected up drift, indicative of a fluvial dominated delta. A similar sequence was observed on the Simeto River delta in eastern Sicily, where during the prograding phase of the delta, the river mouth also developed a cuspate shape and was deflected up drift, compared to the receding phase when the mouth become more arcuate and deflected down drift (Longhitano and Colella, 2007). After 1948, the New Brazos delta also began to retrograde, and in the decades following the 1960s, as the shoreline of the new delta retreated at the mouth, the western flank of the delta was prograding (green areas, Fig. 1d). Historical satellite and aerial imagery since 1995 (Google Earth 7.1, 2013) shows relative stability across the entire subaerial delta. This suggests that presently the delta might have reached a new equilibrium condition.

From these shoreline and general deltaic configuration changes over time, we have organized the evolution of the Anthropocene Brazos delta into three growth phases: (1) the Old Mouth (OM) prior to the diversion, but with most of the changes resulting from jetty construction at the old mouth near the turn of the century, (2) the New Mouth (NM) growth phase that began following the diversion and continued until approximately the 1960s when progradation on the western flank of the new delta increased, initiating (3) the Western Flank (WF) growth phase. Fig. 1e shows these three growth phases as variations in shoreline position relative to the 1852 shoreline position that will serve as our baseline for the pre-Anthropocene configuration. We recognize that there are uncertainties associated with representing past shoreline positions, in particular with the 1852 shoreline, but the general trends of shoreline change depicted in the figure are the primary focus. While the changes to the OM, and the initiation of the NM were the result of impacts directly to the river mouth, the subsequent retreat of the NM starting in the 1950s is not attributable to similar direct changes in the coastal zone. Rather, this period of shoreline retreat was likely the result of the combined effects from drought and upstream human activities within the watershed. Fig. 2 shows average annual water discharge and annual sediment load overtime for the lower river. In the late 1930s and early 1940s the sediment load in the river peaked, increasing from
30 Mt/yr to 40–50 Mt/yr in the 1940s. This is concurrent with the rapid progradation of the NM. In the 1950s however, both sediment load and water discharge dramatically fell to o10 Mt/yr for the sediment load, and to
100 m3/s from
300 to 400 m3/s for the water discharge. This rapid decrease in water discharge and sediment load were the result of a prolonged drought that occurred from 1948 to 1957 (Norwine and Bingham, 1985) termed “The Little Dust Bowl” (Opie, 1989). As the drought relaxed toward the end of the 1950s, the river discharge returned to the pre-drought flow rate, and while the sediment load also increased, it remained about 20% of the pre-drought load at 10 Mt/yr. This reduction in sediment load following the drought was likely the result from a combination of human activities within the watershed that reduced sediment availability. These included changes in agricultural practices following the Dust Bowl in the 1930s (Seelig and Sorensen, 1973), the reduction of non-hay producing cropland, and the conversion of cropland to pastureland (Dunn and Raines, 2001). Coincident with these changes, dam construction created the two largest impounds within the drainage basin, located along the main stem of the river (Dunn and Raines, 2001). As the sediment load stabilized

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to
10 Mt/yr over then next few decades, the delta reached a new equilibrium favoring longshore drift over fluvial supply, resulting in sediment accumulation downdrift (west of the mouth) and leading to the WF growth phase.

3. Material and methods To investigate the record of these changing growth phases in the subaqueous delta stratigraphy, high-resolution geophysical surveys were conducted in 2011. Side scan sonar (SSS), and swath bathymetric data (5 cm vertical resolution) were collected concurrently on two cruises, 10–13 February 2011 and 15–17 March 2011, using a 200 kHz Teledyne Benthoss C3D-LPM High-Resolution Side-Scan Sonar Bathymetric System. Survey transects were oriented parallel to the shore, spaced 100 m, constrained by the 3 and 10 m isobaths based on NOAA nautical chart 11321. The survey covered a total study area of approximately 150 km2 centered on the current (new) river mouth (Fig. 3a). The bathymetric data were corrected to mean low water using NOAA tide station 8772447 located approximately 10 km from the study area. The SSS backscatter data were normalized in order to enhance contrast over relatively small changes in backscatter. Subbottom data were collected using an Edgetechs 216 Full Spectrum Sub-bottom CHIRP seismic sonar operating on frequencies between 2 and 16 kHz towed behind the vessel on 6 June 2011. Five sub-bottom survey transects were oriented shore-normal spaced throughout the study area. Two additional transects were oriented shore-oblique extending from the river mouth to the southwest and northeast respectively (Fig. 3a). To ground truth the geophysical data, 60 surface sediment grab samples were collected on 8 July 2011. In the time between the geophysical surveys and the surficial sediment collection there were no significant wave events (average wave heights were consistently less than 2.5 m and generally less than 2 m), and no significant discharge events from the river (discharges were consistently and significantly below average for this time period). We conclude therefore, that given the lack of any significant events, the surficial sediment characteristics in July 2011 likely represent the characteristics in February and March 2011 when the geophysical surveys were conducted. Particle size distributions for these samples were determined using a Malvern Mastersizer 2000s laser particle diffractometer. The fraction of sand (particle sizes 2000–63 μm), silt (particle sizes 63–4 μm) and clay (particle sizes o4 μm) by volume were determined for each sample from the particle size distribution curves generated during analysis. Prior to analysis, sediment sub-samples were homogenized, combined with a dispersant, and sonicated to prevent and breakup flocs.

4. Results 4.1. Surficial characteristics: side scan sonar data, grain size, and swath bathymetry The SSS data are shown in the mosaic in Fig. 3a. We observed three general backscatter characteristics throughout the study area: low, high, and moderate backscatter. Low backscatter areas were found in nearshore areas that were characterized by consistently low-backscatter throughout, and the area predominantly downdrift of the mouth characterized by small, isolated areas of high-backscatter within a dominantly low-backscatter region. Much of the study areas seaward and updrift (east) of the mouth were characterized by high-backscatter that consisted of large, clearly defined high-backscatter features. The southeastern

Fig. 3. Subaqueous delta study area. (a) Side scan sonar mosaic with the location of the CHIRP subbottom profiles (yellow) shown. In the mosaic lighter colors correspond to high-backscatter. The three distinct backscatter areas observed and discussed in the text are separated by the dashed white lines and labeled. (b) Side scan sonar mosaic with surface sediment percent sand contours overlaid. The locations of the sediment grab samples are shown as a white “X”. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

portion of the study area was characterized as moderate backscatter, as it generally exhibited intermediate backscatter levels, and lacked clearly defined features. Grain-size data from the surface sediment grab samples were collected to ground truth the SSS data. The surface sediment consisted of sand and mud with no gravel. The sand content in the surface sediment is shown in Fig. 3b as contours overlain on the SSS mosaic to evaluate the relationship between the sand content and backscatter. From the figure, areas with a higher percentage of sand ( 460%) are associated with the low-backscatter areas (nearshore areas and areas downdrift of the new mouth). High backscatter areas generally had low sand content ( o20%), with slightly more sand (20–60%) in the moderate backscatter areas. This suggests an inverse relationship between sand content and backscatter for the subaqueous delta. The bathymetric results have been previously discussed in Carlin and Dellapenna (2014), in which a general asymmetry was observed across the delta with generally increased slopes and

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Fig. 4b we see that the most significant erosion (negative change) corresponded to the high-backscatter areas, while depositional areas (positive change) were observed primarily in the low-backscatter area downdrift of the mouth, and the nearshore areas west of the river mouth. The moderate backscatter areas were largely evaluated from the 1982 data set, and showed no net change to slight deposition. Herein, we will refer to these areas that exhibited minimal net bathymetric change as ‘stable’. In total, these results suggest a positive relationship between erosion and backscatter. 4.2. CHIRP subbottom profiles

Fig. 4. Side scan sonar mosaic with bathymetric data. (a) Bathymetric contours from data collected during this study overlaid on the side scan sonar mosaic using a 2 m contour interval. The inset shows the bathymetric relationship with the highbackscatter feature observed near the mouth. A 1 m contour interval is used in the inset. (b) The change in bathymetry from 1938 (solid lines) and 1982 (dashed lines) to 2011 overlaid on the side scan sonar mosaic. Positive numbers indicate net deposition, and negative numbers are net erosion. The locations of CHIRP profiles (yellow lines) discussed in the text are also included. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

depths in areas updrift of the mouth. Fig. 4a shows the measured bathymetric contours overlaid on the SSS mosaic to highlight the relationship between bathymetry and backscatter characteristics. For example, the low-backscatter areas downdrift of the mouth corresponds to a low gradient platform extending southwest from the river mouth, and high-backscatter features were often bounded on their upslope side by discrete isobaths. This is best seen near the mouth where the most clearly defined high-backscatter feature within the study area is bounded by the 5 m isobath directly adjacent to the river mouth, and the 6 m isobath on either side (inset Fig. 4a). The bathymetric data collected during this study were compared to data collected in 1938 and 1982 to illustrate changes in seabed elevation over time. Fig. 4b shows the depth changes as contours (2011–1938 solid contours, 2011–1982 dashed contours) overlain on the SSS mosaic. The extent of each comparison was restricted to the areas where the data sets overlapped. From

The subbottom profiles (Figs. 5–7) generally exhibited 2–4 subbottom units that varied spatially across the study area. The lowermost unit (U0) however, was the only unit consistent throughout all of the profiles. This unit was the thickest, and in most profiles consists of a series of parallel reflectors that are relatively tightly spaced and horizontally positioned. Fig. 5 shows the profiles from updrift of the river mouth. Profile A–A′ was the easternmost profile in the study area. In the landward portion of the profile U0 was overlain by units U1A and U1B. The U1A unit was thin, with seaward-dipping reflectors, and was predominantly located in the middle of the profile. The dip angle of these reflectors was
0.1°. By comparison, U1B was located landward of U1A, was thicker, but thinned seaward exhibiting a noticeable scarp (labeled in Fig. 5), and the internal reflectors were not as clearly defined as in U1A. In the seaward portion of the profile, U1A and U0 were overlain by U2A that thickened in the seaward direction, and contained seaward-dipping (
0.1°) internal reflectors. In the B–B′ profile shown in Fig. 5, U1A was smaller than what was observed in the A–A′ profile, and unlike the A–A′ profile this unit was not exposed at the seabed. In the landward portion of the profile, U1A was overlain by U2B. This unit was similar to U1B as it was thicker landward and thinned seaward with a noticeable scarp, and it also lacked clearly defined internal reflectors. Seaward of U2B, U1A was overlain by U2A with similar characteristics that were observed in the A–A′ profile including seaward-dipping internal reflectors. In line B–B′ U2A also increased in thickness downslope, and reached a maximum thickness in
11–12 m water depths. Fig. 6 shows a profile extending seaward from slightly downdrift of the river mouth. In this profile (C–C′) the U1 units were not observed, and U0 was predominantly overlain by U2A. The internal reflectors within U2A were also seaward dipping as was observed in the profiles updrift of the mouth. Overlying U2A in the landward portion of the line was U2B, which similar to U2B was observed updrift of the mouth, thinned seaward with a noticeable scarp (labeled in Fig. 6). Upslope of the scarp, U2B was overlain by U3B. Fig. 7 shows profiles from downdrift of the mouth. Profile D–D′ is oriented shore-normal extending across the study area. In this profile U2A overlied U0, but large portions of the record appear to be obscured by biogenic gas in the sediments. Overlying U2A in the landward portion of the profile was U3B. This U3B unit also does not exhibit clearly defined internal reflectors similar to U1B and U2B. Seaward of U3B, the U2A and the gas wipeout zone were overlain by U3A. This unit was of a consistent thickness down slope, and exhibited seaward dipping (
0.1°) internal reflectors. Profile E–E′ is oriented oblique to shore extending from the river mouth to the southwest along the low gradient platform/ low-backscatter downdrift areas. In this profile U0 was overlain by U2A, which was overlain by U2B landward, and U3A seaward. The U2B unit was overlain by U3B.

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Fig. 5. Subbottom profiles with interpretations from updrift of the river mouth. The locations of each profile are shown in Figs. 3a and 4b. In the profiles Unit 0 (U0) is interpreted to be relict shelf strata, Unit 1A (U1A) are bottomset beds associated with the Old Mouth growth phase, Unit 1B (U1B) are foreset beds associated with the Old Mouth growth phase, Unit 2A (U2A) are bottomset beds associated with the New Mouth growth phase, and Unit 2B (U2B) are foreset beds associated with the New Mouth growth phase. The white dashed line indicates the surface multiple, which obscured the data below (dark gray area). Vertical distances are estimated using a 1500 m/s sound velocity. The approximate corresponding change in bathymetry results (red – net erosional, blue – net depositional, yellow – stable), and side scan sonar backscatter characteristics (LB – low-backscatter, HB – high-backscatter, MB – moderate backscatter) are noted at the top of the interpretation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5. Discussion

5.1. Unit 0 (U0)

The results in this study provide us with a stratigraphic framework with which we can infer the past processes that have shaped the Brazos River subaqueous delta, and potentially other similar deltas. We will begin with our interpretations of the subbottom units. These interpretations were based on the geometry of the unit, the location vertically within the overall sequence, and the location relative to the river mouth (past and present). After establishing this stratigraphic framework, we will use the surficial data to add insight to the processes that have shaped the delta as it relates to the subaerial delta growth phases.

This unit was the lowermost unit, and the only unit observed in all of the profiles. The base of the unit was obscured by the surface multiple, but it was generally the thickest unit observed. This unit is interpreted to be the relict shelf strata, likely Pleistocene or early Holocene in age, which formed as sea level rose following the last glacial maximum prior to the Brazos delta becoming established in the region. In this sense, this unit represents the pre-delta sediments. The top of U0 is a prominent reflector serving as the base of the deltaic sequences above. This interpretation is consistent with previous seismic data from the subaqueous delta that exhibit a

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Fig. 6. Subbottom profile with interpretations from proximal to the river mouth. The location of the profile is shown in Figs. 3a and 4b. In the profile Unit 0 (U0) is interpreted to be relict shelf strata, Unit 2A (U2A) are bottomset beds associated with the New Mouth growth phase, Unit 2B (U2B) are foreset beds associated with the New Mouth growth phase, and Unit 3B (U3B) are foreset beds associated with the Western Flank growth phase. The white dashed line indicates the surface multiple, which obscured the data below (dark gray area). Vertical distances are estimated using a 1500 m/s sound velocity. The approximate corresponding change in bathymetry results (red – net erosional, blue – net depositional, yellow – stable) and side scan sonar backscatter characteristics (LB – low-backscatter, HB – high-backscatter, MB – moderate backscatter) are noted at the top of the interpretation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

clear distinction between pre-delta (Pleistocene) sediments (Bartek et al., 1990), sediment cores collected from the subaqueous delta that showed Pleistocene sediments overlain by
1–6 m thick deltaic sediments (Rodriguez et al., 2000) and CHIRP subbottom data from the shoreface of barrier islands located updrift of the delta (Carlin et al., 2015; Robb et al., 2003). 5.2. Unit 1 (U1A and U1B) These units, where present, were directly overlying U0, but were only observed in the updrift portion of the study area proximal to the old river mouth. As such, we have interpreted these units to be associated with the OM growth phase (pre-1929). With U1A located seaward of U1B and exhibiting gently seawarddipping internal reflectors, as seen in A–A′ (Fig. 5), we interpret U1A to be the bottomset deposits of the OM growth phase, and U1B to be foreset deposits. In A–A′ there appears little, if any, overlap of these subunits. We believe this is the result of erosion, with the scarp associated with U1B representing the landward extent of the most significant erosion. As such, any foreset deposits seaward of this scarp overlying the bottomset deposits would have been eroded away. In the B–B′ profile (Fig. 5), we only observe U1A. While this may also be the result of erosion completely removing the U1B in the area, given the increased distance of this profile from the old mouth we believe it is more likely that only bottomset sediment from the OM growth phase was deposited in this distal areas from the old mouth. 5.3. Unit 2 (U2A and U2B) These units were observed overlying both U0 and U1A updrift of the new mouth (Fig. 5) and directly overlying U0 downdrift of the new mouth (Figs. 6 and 7). Given the relative location of these units across the delta and within the sequence, we have interpreted these to be associated with the NM growth phase (1929–

1960s). Similar to the OM units (U1A and U1B), we are interpreting U2A to be bottomset deposits, and U2B to be foreset deposits. This is consistent with the observations of U2B being found landward of U2A, and only in those profiles proximal to the new river mouth. For example, in profiles A–A′ (Fig. 5) and D–D′ (Fig. 7) only U2A was observed, as these profiles were located distal to the new mouth. In the areas immediately downdrift of the new mouth (C– C′ Fig. 6 and E–E′ Fig. 7), portions of U2B overlaid U2A, suggesting delta progradation. In B–B′ (Fig. 5) updrift of the new mouth, there is little to no overlap of the subunits similar to the OM units, and there is also a scarp at the seaward extent of U2B. As was the case with A–A′ we believe this also suggests erosion in this portion of the delta, but this would be erosion of the NM delta as opposed to the OM delta in A–A′. Evidence of progradation and erosion is consistent with observations of the subaerial delta, for example the shoreline progradation from 1929 to 1948, and retrogradation following 1948 during the NM growth phase. 5.4. Unit 3 (U3A and U3B) These units were only observed downdrift of the new river mouth, and overlaid NM units (U2A and U2B). In the far western end of the study area these units directly overlaid U0 (figure not included). As a result, we are interpreting these units to be associated with the WF growth phase (post-1960s). As with the other units, U3A is interpreted as bottomset deposits with U3B being foreset deposits. In profile E–E′ (Fig. 7) that was oriented obliquely to shore, there was some overlap of U3B on U3A suggesting progradation, but little if any in profile D–D′ oriented shore-normal. This is consistent with the subaerial shoreline changes that suggest that during the WF growth phase delta progradation was more alongshore with longshore drift than seaward. Additionally, erosional scarps, if any, were not as clearly defined in these surface units compared to OM and NM surface units from updrift of the new mouth suggesting minimal erosion or net deposition.

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Fig. 7. Subbottom profiles with interpretations from downdrift of the river mouth. The locations of each profile are shown in Figs. 3a and 4b. In the profiles Unit 0 (U0) is interpreted to be relict shelf strata, Unit 2A (U2A) are bottomset beds associated with the New Mouth growth phase, Unit 2B (U2B) are foreset beds associated with the New Mouth growth phase, Unit 3A (U3A) are bottomset beds associated with the Western Flank growth phase, and Unit 3B (U3B) are foreset beds associated with the Western Flank growth phase. The white area in Profile D–D′ prime is believed to be data obscured by biogenic gas produced in the sediments. The white dashed line indicates the surface multiple, which obscured the data below (dark gray area). Vertical distances are estimated using a 1500 m/s sound velocity. The approximate corresponding change in bathymetry results (red – net erosional, blue – net depositional, yellow – stable), and side scan sonar backscatter characteristics (LB – low-backscatter, HB – highbackscatter, MB – moderate backscatter) are noted at the top of the interpretation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For the surficial characteristics, the SSS data proved somewhat difficult to interpret. The relationship of backscatter to sediment texture showed high-backscatter areas were generally muddominated, while the sandy areas generally had moderate to lowbackscatter (Fig. 3b). This relationship of backscatter to sediment texture is counterintuitive given that SSS backscatter is typically positively correlated with mean grain size (e.g. Collier and Brown, 2005). One reason for this inverse relationship between grain size and backscatter may be due to the fact that backscatter data were normalized to accentuate changes over a relatively small range of values. While this adds difficulty in definitively interpreting

surficial sediment characteristics from the SSS data, qualitatively what is clear from these data is that the backscatter characteristics are consistent with erosional/depositional characteristics on the subaqueous delta. The high-backscatter areas were consistent with net erosion, moderate and low-backscatter areas were stable or net depositional respectively (Fig. 4b). Therefore, we will focus primarily on net erosional versus depositional characteristics as it relates to the subaqueous delta stratigraphy, using the SSS data as a means to quickly visualize the spatial extent of the dominant process. Combining these erosional and depositional characteristics

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Fig. 8. Interpretation map of the subaqueous delta based on the results from this study. The map delineates areas that have been dominantly net erosional from areas that have been dominantly net depositional and stable (minimal or no net change) over the Anthropocene history of the delta. Solid colors represent the interpretation of the exposed surface units as it relates to the subaerial growth phase, Old Mouth (OM), New Mouth (NM), and Western Flank (WF). The dashed colored lines show the approximate buried extent of the OM bottomset (red) and NM bottomset (blue) units. Past shoreline configurations (solid lines); 1933 (red), 1948 (blue), and 1971 (green), are included for reference. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

from the surficial data to the subbottom data we see that net erosional areas typically corresponded to those areas with earlier growth phases (OM or NM) exposed at the surface. In Figs. 5–7 the corresponding SSS backscatter characteristics, and relative change in bathymetry (net erosional, net depositional, stable) are shown at the top of the interpreted CHIRP profiles. For the updrift profiles (Fig. 5, A–A′ and B–B′), we see that the majority of these areas were net erosional. The exception is in profile B–B′, where the seaward-most portion of the profile was generally stable. In both of these profiles however, the landward extent of the high-backscatter corresponded to the base of the erosional scarps. Seaward of these scarps corresponded to areas with the highest values of net erosion (Fig. 4b). For profile A–A′ in this area net erosion is 4– 5 m, also corresponding to the thinnest deltaic sequence (
1 m or less in areas). Furthermore, this is the only area with OM units exposed at the surface. This suggests that the depocenter shifted with the changing growth phases, and erosion removed portions of the deltaic sequence at the abandoned depocenter location, leaving the older, earlier deposited sediment exposed at the surface. By contrast, the downdrift transects (Fig. 7, D–D′ and E–E′) were exclusively net depositional, low SSS backscatter, and WF units were exposed at the surface. These areas exhibited the thickest deltaic sequences (44 m in some areas), consistent with net deposition. The mouth transect (Fig. 6, C–C′) was net depositional at the landward extent, corresponding to low-backscatter and NM and WF exposed units, and was net erosional seaward of the erosional scarp transitioning to stable at the seaward extent. This area seaward of the erosional scarp was also characterized by high SSS backscatter similar to the updrift profiles. Overall, these data show a clear trend along the delta. Moving from updrift (east) to downdrift (west), the delta shifts from being erosion and high-backscatter dominated, with older growth phase units exposed at the surface to depositional and low-backscatter dominated, with the WF units (the most recent or current growth phase) exposed at the surface. The exception to this trend are the stable areas located at the seaward ends of profiles B–B′ (Fig. 5)

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located directly updrift of the new mouth, and C–C′ (Fig. 6) located directly downdrift of the new mouth. This suggests that either the eroded material from upslope is being transported across the shelf, and deposited within these areas, or the majority of the erosion was concentrated upslope leaving these deeper water areas generally unaffected. Both of these possibilities are consisted with ravinement depth estimates (
8–10 m) for this part of the GOM (Rodriguez et al., 2001). The ravinement depth is the depth were erosional processes can no longer rework coastal sediments (Nummedal and Swift, 1987) and although typically a function of wave energy, the ravinement depth can be raised independent of changes in wave energy, by increasing sediment supply (Rodriguez et al., 2001). Thus, during the active growth phase, the ravinement depth proximal to the sediment source may be raised, but when the growth phase shifts and sediment supply to that area is reduced, the ravinement surface is lowered, and the sediment above is reworked. Portions of the delta that are below the ravinement depth initially however, may remain unaffected by shifts in growth phases and sediment supply. In Fig. 4a the stable areas were observed in water depths below 11–12 m. By being below the ravinement depth, this would allow for both upslope-eroded material to deposit, but also for negligible erosion to occur as a result of ravinement processes. It is likely that a combination of these factors has contributed to relatively stable bathymetry in these areas. These stable areas also corresponded to the deepest water depths measured during the study, which explains why similar characteristics were not observed anywhere else within the study area. From these characteristics we can create a generalized map of the processes and exposed sedimentary units across the study area to depict the shifting depocenters as the delta evolved (Fig. 8). From the map we see that erosional areas (typically those areas with older growth phase units exposed) dominate much of the study area, with depositional areas restricted to downdrift of the new mouth. The stable areas did have NM units exposed at the surface, but were found distal to the new mouth in deeper water, likely beyond the influence of WF deposition and erosion due to ravinement. This variation in erosional areas and depositional areas also explains the bathymetric asymmetry across the subaqueous delta that was observed by Carlin and Dellapenna (2014). Furthermore, this erosional to depositional pattern across the delta is consistent with the observed changes in subaerial growth phases over time. Therefore, erosional areas represent previous depocenters, and the depositional areas are the current depocenter. While this sequence is similar to autocyclic delta evolution (new depocenters activated, abandoned, and eroded), it is not autocyclic as the changes have primarily been the result of the human-induced alterations to the fluvial–deltaic system. The cycles of depocenter activation and abandonment due to anthropogenic forces has been observed in other deltas such as the Danube River delta (Giosan et al., 2013), the Yellow River delta (Li et al., 2000; Qiao et al., 2011; Zhou et al., 2014) and the Rhone River delta (Fanget et al., 2014; Sabatier et al., 2006). It should be noted, that as the depocenters are abandoned and eroded, some of that material is likely incorporated into the subsequent new depocenter. The redistribution of old deltaic sediment to incorporate into the new depocenter on the Brazos delta was mentioned previously by Rodriguez et al. (2000) and is supported by the geochronological results of Carlin and Dellapenna (2014). In this latter study, 210Pb and 137Cs age models could only resolve the past few decades as the delta reached an apparent equilibrium. Prior to that, the age model breaks down due to nonsteady state accumulation, which may be explained by the incorporation of older sediment from a nearby abandoned depocenter. While a volumetric sediment budget could prove useful in determining the fractional contribution of past depocenters to the

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current depocenter, unfortunately the extent of the survey area does not lend itself to such a calculation. In Fig. 4b we see that some of the areas of maximum deposition (2 m) were observed along the southwestern extent of the study area, suggesting that the center of the depocenter may actually be beyond the study area. Therefore any calculation of new sediment in the depocenter would likely be an underestimate. What we can conclude from the data is that the subaqueous delta has preserved the history of these changes in depocenters over time (Fig. 8). For example, OM sediment remains exposed near the location of the 1933 shoreline position, NM sediment near the 1948 position, and WF sediment in the current or most recent depocenter. Fig. 8 also depicts the estimated extent of the units buried by the subsequent growth phase deposits. The extent of the OM bottomset (red dashed line) is located between the old and new mouth, and is generally consistent with the shape of the 1933 shoreline (solid red line). The extent of the NM-bottomset (dashed blue line), buried by the WF units, suggests a larger delta during this time compared to the OM delta, which is consistent with the relative scale of the subaerial delta in 1948 (solid blue line). Furthermore, it may be said that the subaqueous delta has preserved these changes better than the subaerial delta in some areas, in particular with the OM growth phase. From Fig. 1e, we see that by 1960 the OM shoreline had almost completely returned to its 1852 position, suggesting that this portion of the subaerial delta seemingly exhibited minimal net change over time. The preservation of OM associated sedimentary units within the subaqueous delta may in turn be the only lasting evidence of this short-lived progradational phase of the delta. Collectively, this suggests that the subaqueous delta has responded similarly to the subaerial delta in the wake of anthropogenic alterations to the system. The depocenter migrated southwestward through time, new clinoforms became established as the previous ones were abandoned and eroded. While these cycles were similar to what was observed within the subaerial delta, evidence of these cycles may actually be better preserved within the subaqueous delta. Recognizing these sediment characteristics allows one to distinguish the Holocene deltaic sequences from the Anthropocene delta. Understanding how these changes are preserved in the geologic record is important for understanding the evolution of these Anthropocene deltas, which may likely become ubiquitous as deltas globally are becoming increasingly more influenced by human activities.

6. Conclusion The Brazos River delta is an Anthropocene delta, thus influenced by both natural and human-derived forces. This study showed that for this delta the dynamic evolution of subaerial delta equally corresponds to a dynamic evolution in the subaqueous delta. As human activities alter the fluvial–deltaic system, new depocenters are created while previous depocenters are abandoned and eroded. Preserved within the sedimentary record of the subaqueous delta is the history of those changes over time. For the Brazos River delta, we used three progradation phases of the subaerial delta resulting from anthropogenic alterations to the system, to understand the processes that have shaped the subaqueous delta. Progradation during the OM phase occurred after the completion of the jetties in 1897. This action interrupted longshore drift, leading to a rapid shoreline progradation, and the initiation of the first Anthropocene depocenter. The NM phase occurred after the diversion of the mouth, as elevated sediment loads from the river coupled with sediment supplied from the abandoned OM depocenter facilitated the fastest progradation of

the delta. The rapid progradation during this phase resulted in the NM clinoform to prograde over the abandoned OM clinoform in some areas. Changes in the watershed that reduced the river's sediment load were accentuated by a prolonged drought. This event caused the NM delta to retrograde, subsequently initiating the most recent WF progradational phase. The depocenter shifted westward again as the NM depocenter was abandoned and eroded. The WF clinoform continued to prograde over the abandoned NM clinoform until equilibrium was reached. For the subaqueous delta, these cycles of depocenter initiation, abandonment, and erosion are preserved within the sedimentary record. In some areas the preservation of these past events may only be preserved within the subaqueous delta, as the shoreline and subaerial deltas returned to their pre-Anthropocene configurations. This study showed that as we move forward in the Anthropocene, more deltas may, or already have transitioned to Anthropocene deltas, resulting in rapid and dramatic changes across the subaerial and subaqueous delta as the new equilibrium is established. The key to understanding the evolution of these deltas may lie within the subaqueous portions, where the most complete record of the past stratigraphic changes may best be preserved.

Acknowledgments The authors would like to thank the crew of the R/V Manta, Joshua Williams, Timothy Eyerdom and others that assisted with the data collection. Additionally we would like to thank Peter van Hengstum and two anonymous reviewers for the extremely helpful comments on earlier drafts of the manuscript. This paper is funded in part by a grant/cooperative agreement from the National Oceanic and Atmospheric Administration Award No. NA09NOS4190165. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its subagencies.

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