Journal of Coastal Research
SI
85
381–385
Coconut Creek, Florida
2018
Palynological and Geochemical Records of Environmental Changes in a Taxodium Swamp near Lake Pontchartrain in Southern Louisiana (USA) during the Last 150 Years Junghyung Ryu†*, Thomas A. Bianchette †, Kam-biu Liu †, Qiang Yao†, and Kanchan D. Maiti † † Department of Oceanography and Coastal Sciences, College of the Coast and Environment, Louisiana State University, Energy, Coast & Environment Building, Baton Rouge, LA 70803, USA, jryu1@ lsu.edu
www.cerf-jcr.org
ABSTRACT Ryu, J.; Bianchette, T.A.; Liu, K.-B.; Yao, Q., and Maiti, K.D., 2018. Palynological and Geochemical Records of Environmental Changes in a Taxodium Swamp near Lake Pontchartrain in Southern Louisiana (USA) during the Last 150 Years. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 381–385. Coconut Creek (Florida), ISSN 0749-0208. www.JCRonline.org
The hydrological and environmental histories of estuarine wetlands in southern Louisiana are not well-documented. To better understand their local developmental processes, a 59 cm sediment core (WMA-1) was extracted from a bald cypress (Taxodium distichum) swamp located approximately 800 m west of Lake Pontchartrain. This area has been heavily influenced by human activities, and is susceptible to inundation from storm surges from Lake Pontchartrain as well as fluvial and lacustrine processes. 210Pb analysis indicates that WMA-1 has a sedimentation rate of 0.39 cm/year, consistent with the detection of a 137Cs peak at 18 cm depth. Sedimentological and geochemical analyses reveal two distinct sedimentological facies: an organic-rich dark brown peat unit from 0 to 29 cm containing low concentrations of terrestrial elements (e.g., Ti, and Fe), and a clay unit from 29 to 59 cm with elevated concentrations of most elements (e.g., Sr, Cl, Ti, Mn, and Fe). The pollen assemblage in the clay section is dominated by TCT (mainly Taxodium), but it is replaced by Salix and wetland herbaceous taxa in the overlying peat. The Taxodium swamp was present at the site at least 150 years ago, and was replaced by Salix and other bottomland hardwood trees and wetland herbs due to a decrease in water level around AD 1940. While climate may have played a role, this hydrological change was most likely caused by the significant regional human activities during the last 100 years. Two thin clay layers (at 3-5 cm, 14-19 cm) embedded in the upper peat section were likely deposited from hurricanes Isaac (2012) and Betsy (196): two events responsible for heavy rain and significant storm-surge flooding. Our findings suggest that human activities are capable of altering local wetland hydrology and ecology, while the clay layers represent hurricane-induced floods at the site during the past century.
ADDITIONAL INDEX WORDS: environmental history, estuarine wetlands, sediment, pollen, XRF, loss-onignition, Taxodium, hurricanes, human activities.
INTRODUCTION Bald cypress (Taxodium distichum) swamps serve as important ecosystems in the increasingly vulnerable wetlands of southern Louisiana. Specifically, T. distchum swamps offer protection from intense flood and wind events, and unique shelters for freshwater organisms (Williams et al., 1999). These ecosystems are predominant in the freshwater forested wetlands located near Lake Pontchartrain (Lopez, 2003), where these wetlands form one of the largest estuaries in the United States (Keddy et al., 2007). Due to the depressed oval shape of Pontchartrain Basin, the adjacent Taxodium swamp forests are prone to rapid and dynamic hydrological and geomorphological changes. These changes have been attributed to various natural and anthropogenic impacts (Keddy et al., 2007; Shaffer et al., 2009). Natural impacts include the evolution of the Mississippi Delta lobes (Roberts, 1997), sea level rise (Glick et al., 2013), subsidence (Shaffer et al., 2009), saltwater intrusion (Thomson et al., 2001), and hurricanes (Liu et ____________________ DOI: 10.2112/SI85-077.1 received 30 November 2017; accepted in revision 10 February 2018. *Corresponding author:
[email protected] © Coastal Education and Research Foundation, Inc. 2018
al., 2015). Human activities can include oil and gas extraction, road construction, dredging, and flood control projects such as levees, spillways, canals, and dams that threaten the integrity of the wetlands (Keddy et al., 2007). Regional published wetland studies largely focus on event deposition (Liu et al., 2015), histories of ecosystems (Shaffer et al., 2009), or wetland ecology (Keddy et al., 2007). However, they do not generally focus on decadal-scale hydrological and ecological changes. Decadal-scale investigations are important for better understanding how coastal wetlands will respond to future environmental impacts, possibly caused by climate change, human disturbance, and hydrological alterations. A longer-term perspective can best be attained by using a paleoecological approach to reveal the ecological responses of wetlands to hydrological changes and anthropogenic activities in the past. For this study, we used multiple proxies to reconstruct the anthropogenic and natural impacts on a Taxodium swamp located near Lake Pontchartrain. STUDY SITE The Maurepas Swamp Wildlife Management Area (MSWMA), located between Lake Pontchartrain and Lake Maurepas, is dominated by bald cypress (Taxodium distichum)
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_________________________________________________________________________________________________ 5, 7, 12, 18, 22, 27, 30, 36, 46, 52, 58 cm) were processed for pollen analysis using conventional methods (Liu et al., 2008). At least 300 pollen grains were counted for each sample. Published pollen guides (McAndrews et al., 1973; Willard et al., 2004) were used to aid identifications. Charcoal fragments > 10 μm were counted. 210Pb and 137Cs measurements were determined by conventional methods (Maiti and Benitez, 2010) . An age model (Fig. 2) was constructed from the radionuclide measurements. Vertical accretion rate was determined from the 210Pb and 137Cs measurements, with the assumption that bioturbation is negligible. RESULTS Core Stratigraphy Core WMA1 contains two distinct sedimentary facies, separated by a sharp contact. Zone 1 (29-59 cm) is dominated by grey clay (31-67% water content, 4-19% organics, 1-3% carbonates, and 48-95% residual material) (Fig. 3). At 29-34 cm and 43-52 cm, two light-grey bands are present, containing relatively low organic (4-11%) and water contents (30-50%). At 34-43 and 52-59 cm, dark-grey clay is dominant, containing relatively high organic (8-17%) and water (44-56%) contents. Zone 2 (0-29 cm) consists of dark-brown peat with 67-87% water content, 19-50% organics, 1~5% carbonates, and 48–77% residual material (Fig. 3). Light-brown clay bands are present at 3–5 cm and 15–19 cm, containing relatively low organic contents (21-25% and 20-23%, respectively).
Figure 1. Map of study site. WMA-1 coring site is indicated with a star (A). The proximity of the coring site to Lake Pontchartrain, Lake Maurepas, and local roads and railway is shown (B). Study site maps (A) and (B) were produced using ArcGIS 10.3.
and tupelo (Nyssa) forests, and bottomland hardwood forests. The majority of MSWMA swamps have been degraded due to a combination of sea level rise and subsidence (Thomson et al., 2001). For this study, we focus on a relic swamp (Shaffer et al., 2009) dominated by Taxodium-Nyssa forest. The area is subjected to seasonal flooding of fresh and brackish water, leading to a collection of freshwater to oligohaline environments. METHODS In February 2013, core WMA-1 (30°06'54.94"N, 90°26'12.16"W) was extracted from a small shallow pond (
