Examining 239+240Pu, 210Pb and historical events to determine carbon, nitrogen and phosphorus burial in mangrove sediments of Moreton Bay, Australia
Introduction
From the 238U decay series, the 210Pb dating method is the most widely used radionuclide tracer to form geochronologies from the previous 100 years (Mabit et al., 2014, Sanchez-Cabeza and Ruiz-Fernández, 2012). This sediment dating method is particularly useful to interpret the effects of climate change (Chen et al., 2014, Mabit et al., 2014), sea level rise (Gehrels et al., 2012, Sanders et al., 2012, Smoak et al., 2013) and in studies determining environmental impacts during the previous century (Figueiredo et al., 2013, Harmon et al., 2014, Raygoza-Viera et al., 2014, Reinemann et al., 2014, Sanders et al., 2006). Indeed, the 210Pb dating method has been used to date coastal sediments for more than 40 years (Koide et al., 1973), including carbon burial rates in coastal wetlands (Sanders et al., 2010a, Weston et al., 2014). In the Northern Hemisphere anthropogenic radionuclides from atmospheric fallout following nuclear testing have been widely used as a tracer of sedimentation processes, however, this approach has not been widely used in the Southern Hemisphere (Everett et al., 2008, Leslie and Hancock, 2008, Sanders et al., 2014a, Santos et al., 2008, Tims et al., 2010). Some authors point out that the input of 137Cs in the Southern Hemisphere was quite small, thereby limiting the use of this tracer for dating purposes, although there exist several studies that have been done using anthropogenic fallout radionuclides in Southern Hemisphere e.g. (Everett et al., 2008, Sanders et al., 2006, Tims et al., 2010). Even with the approximately fourfold lower inventories of bomb-test fallout (Kelley et al., 1999, Ketterer et al., 2004, Sanders et al., 2010b), advances in technology now allow accurate measurements of the low anthropogenic Pu activities in the coastal sediments of the Southern Hemisphere (Everett et al., 2008, Leslie and Hancock, 2008, Sanders et al., 2014a, Sanders et al., 2014b, Sanders et al., 2010b, Tims et al., 2010, Tims et al., 2013). Plutonium-239 and 240Pu, with half-lives of 24,110 and 6561 years respectively (http://www.nucleide.org/DDEP.htm), may be used as a marker of sediment deposited since the early 1950s. Plutonium is relatively immobile in both freshwater and saltwater conditions and is efficiently scavenged by sediment particles (Ketterer et al., 2004). These traits along with a well-defined fallout record make 239+240Pu an ideal tracer of coastal processes and events occurring during the previous 60 years. An increasing number of studies are utilizing excess 210Pb and 239+240Pu as tracers of sedimentation processes, including organic carbon (OC) burial during the previous century (Breithaupt et al., 2012). Because OC burial rates may reflect local conditions, comparative observations are needed to fully understand the drivers of global carbon sequestration in mangrove sediments. In addition, burial rates contribute to understanding potential impacts on this sink capacity due to climate change, local hydrological regimes, and widespread eutrophication (Alongi, 2008). Indeed, the associated nitrogen (N) and total phosphorus (TP) burial in mangrove sediments may partially offset anthropogenic nutrient loads and eutrophication in the increasingly developed coastal regions of the tropics and subtropics (Sanders et al., 2014b). Carbon and nitrogen stable isotope signatures (δ13C and δ15N) may also be used to identify the sources of buried organic matter (Sanders et al., 2014b). The primary objective of this paper was to use a combination of radionuclide tracers, lithostratographic profiling, and stable isotopes to determine how local anthropogenic disturbances influence sediment accumulation dynamics. We hypothesize that the high TP import into a mangrove creek (Eyre et al., 2011) drives an increase in the algal and/or allochthonous material deposition and an increase OC and N burial (Sanders et al., 2014b). A second hypothesis of this work is that stable isotopes of OC and N (δ13C and δ15N), in combination with C/N molar ratios (Eyre et al., 2013), may provide insight into the source of organic material being deposited. These hypotheses were tested by using well-established mangrove sediment dating methods, 239+240Pu and excess 210Pb (210Pbex), and lithological profiling to determine OC, N and TP burial rates.
Section snippets
Study area
This study was conducted in the subtropical mangrove forest in southern Moreton Bay, Australia, just south of Brisbane (population >2 million) (Fig. 1). The study site is a small, well defined mangrove creek with no upstream freshwater input. The tidal regime is semidiurnal with neap tide and spring tide ranges of 1 and 2 m, respectively. The study creek is within Southern Moreton Bay; a semi-enclosed waterbody, with the Brisbane, Logan and Caboolture rivers draining into the bay (see Eyre and
Plutonium-239+240 and 210Pb profiles
The plutonium isotopic ratios (240/239Pu) of ∼0.18 observed in the Moreton Bay mangrove sediments are in agreement with other marine measurements in the Southern Hemisphere (Fig. 2). These ratios are characteristic of stratospheric fallout from nuclear bomb testing beginning in the early 1950s. Plutonium-239 and 240Pu ratios from global fallout (∼0.18) differ from well-known 240/239Pu ratios from events such as Fukushima (0.30–0.33) (Zheng et al., 2012) and Chernobyl (∼0.43) (Ketterer et al.,
Conclusion
Our combination of sediment-dating models provided insight on the dominant processes occurring during sedimentation in mangrove systems. The well-established 239+240Pu and 210Pb dating methods were combined with historical events to interpret coastal sedimentation processes during the previous century. The 210Pbex CRS model supported our hypothesis that high phosphorous import to the tidal creek may increase mangrove OC burial. This hypothesis was further supported by δ15N and C/N molar ratios
Acknowledgment
CJS is funded through a SCU Postdoctoral Fellowship. JLB is funded under STAR Fellowship Assistance Agreement no. F13B20216 by the U.S. Environmental Protection Agency. This manuscript has not been formally reviewed by EPA. The views expressed are solely those of the authors, and EPA does not endorse any products or commercial services mentioned in this publication. We thank Matheus Carvalho for running the C and N samples and the Environmental Analysis Laboratory at Southern Cross University
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