- Email: [email protected]
Organic matter content and particle size modifications in mangrove sediments as responses to sea level rise
Marine Environmental Research 77 (2012) 150e155
Contents lists available at SciVerse ScienceDirect
Marine Environmental Research journal homepage: www.elsevier.com/locate/marenvrev
Short communication
Organic matter content and particle size modifications in mangrove sediments as responses to sea level rise Christian J. Sanders a, *, Joseph M. Smoak b, Mathew N. Waters c, Luciana M. Sanders d, Nilva Brandini a, Sambasiva R. Patchineelam a a
Universidade Federal de Fluminense (UFF), Departamento de Geoquímica, Niterói-RJ, Brazil University of South Florida (USF), Environmental Science, St. Petersburg, FL, USA Valdosta State University, Biology Department, Valdosta, GA, USA d Universidade Federal do Rio de Janeiro, Laboratório de Biogeoquímica, Rio de Janeiro - RJ, Brazil b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2011 Received in revised form 26 November 2011 Accepted 8 February 2012
Mangroves sediments contain large reservoirs of organic material (OM) as mangrove ecosystems produce large quantities and rapidly burial OM. Sediment accumulation rates of approximately 2.0 mm year1, based on 210Pbex dating, were estimated at the margin of two well-developed mangrove forest in southern Brazil. Regional data point to a relative sea level (RSL) rise of up to w4.0 mm year1. This RSL rise in turn, may directly influence the origin and quantity of organic matter (OM) deposited along mangrove sediments. Lithostratigraphic changes show that sand deposition is replacing the mud (<63 mm) fraction and OM content is decreasing in successively younger sediments. Sediment accumulation in coastal areas that are not keeping pace with sea level rise is potentially conducive to the observed shifts in particle size and OM content. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Organic matter Deposition rate 210 Pb Dating Sedimentation rate Mangroves
1. Introduction The (sub)tropical coastal ocean shorelines of the world are widely lined with mangrove forests. Mangrove forest provide numerous ecosystem services as well as produce and retain large quantities of organic matter (OM) within their sediments and play an important role in coastal nutrient cycling (Perry et al., 2008; Sanders, 2010a: Ferreira et al., 2010). Also, mangroves efficiently trap fine sediments by slowing water movement to speeds conducive for particle deposition (Cahoon and Lynch, 1997; Young and Harveya, 1996). Mangrove sediments are influenced by many factors including daily changes in tidal energy, temperature, salinity and varying levels of anoxia (Alongi, 2008). The sedimentation processes in mangrove habitats are ultimately controlled by local hydrodynamics, including relative sea level (RSL) oscillations, tropical storms and sediment supply (Cahoon and Lynch, 1997; Lynch et al., 1989; Sanders, 2008; Alongi, 2008). The relationship between climate change and RSL fluctuations is an important issue globally (Hansen et al., 2005; Meehl et al., 2005).
* Corresponding author. Tel.: þ55 021 2629 2200. E-mail address: [email protected] (C.J. Sanders). 0141-1136/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.marenvres.2012.02.004
The Intergovernmental Panel on Climate Change (IPCC, 2007) has projected that the rate of global sea level rise, consequent to climate change, may increase up to 5.9 mm year1 by 2100. Regional variations in the RSL rates are found along the coastal regions of the world ocean (Pikey and Cooper, 2004) and have been shown to cause a direct response in mangrove forest sedimentation (Gilman et al., 2007). Kim et al. (2005) demonstrated mangrove displacement and lithostratigraphic profile modifications over approximately 20 kyr time period. Using stratigraphic sequences in mangrove peat deposits, Woodroffe (1990) concluded that mangroves may progressively move inland within decadal timescales. Indeed, many mangrove margins are experiencing considerable loss as a result of forest setback directly influenced by increased sea level rise (Lopez-Medell et al., 2011; Krauss et al., 2011). For example, in South Florida, mangrove forests have migrated inland by 1.5 km since the mid-1940s (Ross, 2000). Other studies have shown that mangrove sediment accretion is capable of keeping pace with sea level rise, not affecting the lithostratigraphic profiles (Lynch et al., 1989; Sanders, 2008, 2010b). The goal in this investigation is to examine sediment accumulation rates and to determine the time-interval changes in the ecological and depositional history (grain size, OM, chlorophylla and pheo-pigment) at Rhizophora spp. dominant margins of two
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
well-developed mangrove forests in southern Brazil. Here we show how RSL rise is causing a decrease in the OM deposition in mangrove forests where sediment accumulation rates are not keeping pace with RSL rise. While entire forests might shift landward, at these particular locations, migration is limited by geological constraints (i.e., mountain range), which is the case for the entire South-Southeastern coast of Brazil and therefore could ultimately result in collapse of these mangrove ecosystems. 2. Material and methods 2.1. Study site The mangrove forests of our study are located in Guaratuba Bay (GTA) and Laranjeiras Bay (PAA), southern Brazil (Fig. 1). The two bays are surrounded by well-developed mangrove forests, forming drainage basins off the Serra do Mar mountain range. The study sites of this work were selected based on their location near the latitudinal tolerance were mangrove thrive. Just to the south, these ecosystems are replaced by salt marshes in similar coastal settings. This location is ideal to identify impacts of climate change on mangrove stability. Sediment cores were collected from sites that appeared spatially homogenous without any abrupt transitions or indications of perturbations and therefore representative of the system. Time-series sea levels, as indicated by a nearby tide gauge in the region, show a RSL of w4.0 mm year1 during the past w50 years, while sedimentation accumulation rates in a stationary mangrove forest north of these sites indicate a RSL rise of over 2.5 mm year1
151
during the 20th century (Sanders, 2010b). As in many regions of the world, data to determine RSL trends along the Brazil coast are limited. This is because, unlike many countries in the northern hemisphere, tide gauges in South America have not been utilized consistently enough to determine confident RSL trends; therefore the RSL rates used for comparative reasons in this study are limited. The Laranjeiras Bay estuary is adjacent to Paranaguá Bay, home to one of Brazil’s busiest ports, whereas the estuary of Guaratuba Bay is relatively small with little anthropogenic activity. Although Laranjeiras Bay is near Paranaguá Bay (Fig. 1), this study site is located in an environmentally protected area, w30 km from the port. The specific mangrove study sites were chosen in regions unaffected by direct urban development. The estuaries are situated about w90 km SoutheSoutheast of the industrialized city Curitiba, which is 950 m above sea level. The annual average temperature and precipitation in the region is w21 C and w22 cm respectively. 2.2. Sediment sampling and analyses A sediment core from each site was collected in 2005 by inserting a 7 cm diameter acrylic tube into the substrate during low tide. The sediment was extruded out of the tube and sectioned at 1-cm intervals from the core top to 10 cm mark and subsequently at 2cm intervals to the bottom of the core. Organic matter was determined through loss of ignition (LOI), at 550 C for 2 h. Organic carbon values were estimated from loss on ignition using the conversion quotient of 1.724 (Schumacher, 2002). From the original wet section, an aliquot was taken for determining dry bulk density (DBD), which was estimated by taking into consideration the split’s
Fig. 1. Study area. a) Laranjeiras Bay and Guaranatuba Bay, Brazil; b) illustration of a mangrove margin where samples were taken.
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
a
6.0 5.5
LN(210Pbex)Bq/kg
porosity based on fractional water content and density of the interstitial water, OM and granulometry. Granulometric analyses were done using a CILAS 1064 diffraction laser unit. The chlorophylla and pheo-pigments were analyzed according to Plante-Cuny (1978), using a mod. Shimadzu UV-1601 optic density absorption spectrometer. Pigment content calculation was after Lorenzen (1967) and expressed as mg per g of organic carbon. For radionuclide analyses, the remaining sediment at each interval was sealed in 70 ml petri-dishes for at least three weeks, to establish secular equilibrium between 226Ra and 214Pb. Gamma-ray measurements for the radionuclide activities were achieved by using a semi-planar intrinsic germanium high purity coaxial detector with 40% efficiency, housed in a lead shield, coupled to a multichannel analyzer. Leade210 activity was determined by the direct measurement of 46.5 KeV gamma peak, while 226Ra activity was calculated based on estimates of 214Pb at 351.9 KeV peak (Appleby, 1988). Activities were calculated by multiplying the counts per minute for each radionuclide minus background counts, by a factor that includes the gamma-ray intensity and detector efficiency. This factor was determined from standard calibrations using an efficiency curve obtained by measuring and analyzing a certified standard cocktail of radionuclides; this standard was certified by IRD e (Instituto de Radioproteção e Dosimetria), certified no C/87/A00. The excess 210 Pb (210Pbex) activity was estimated by subtracting the 226Ra from the total 210Pb activity. Each sample was counted for 86,000 s in identical geometrical cylinders. Self-absorption correction was calculated following Cutshall et al. (1982). The sedimentation rates were obtained through the Constant Initial Concentration (CIC) dating method outlined in Appleby and Oldfield (1992).
5.0 4.5 4.0
GTA
3.5 3.0 2.5 0
3
6
9
12
15
18
16
20
24
Depth (cm)
b
5.5 5.0 4.5
LN(210Pbex)Bq/kg
152
4.0 3.5 3.0
PAA 2.5 2.0 1.5 1.0 0
4
8
12
Depth (cm) 210
Fig. 3. Mud - normalized Pbex distribution for sediment cores; (a) GTA and (b) PAA, distributed against depth (cm).
3. Results A net decrease in the 210Pbex activity, down the core depth, was recorded in both profiles (Fig. 2). Sand (>63 mm) content could be an influencing factor in the 210Pbex distribution due to the granulometric thickening towards the top of both cores (Fig. 2). Thus, the 210Pbex was normalized to the contents of the mud fraction. This was done assuming that sand has little or no 210Pbex activity (Ravichandran, 1995). After the correction, in each of the cores the net log profile of 210Pbex was almost linear extending from below the apparently turbated surface layers to the core depth where the parent-supported 210Pb activity was at background level (Fig. 3a, b). The 210Pbex counting errors ranged between 8 and 16%, increasing towards the bottom of the cores. As the net log linear profiles of the
Sand (%) 57
2003 1993 1983
year
1973 1963 1950
37 37 42 38
1930 1910 1890
20
70 67 59 54 61 54 49 55 49 49
26 25
22 34 24 21 18 19 21 18 13 9 7 5
Pbex were linear, the constant initial concentration (CIC) dating method as adopted in this study was justified. The statistics related to the cores’ 210Pbex distribution are as follows; Core PAA (y ¼ 0.16x þ 5.54) (r2 ¼ 0.89; n ¼ 14; p < 0.05) from the 3.5 to the 23 cm interval; Core GTA (y ¼ 0.15x þ 5.65) (r2 ¼ 0.89; n ¼ 14; p < 0.05) from the surface to the 17 cm mark of the cores. The CIC dating method implies that the sediment accumulation rate has been consistent during the time span encompassed in our study (Appleby and Oldfield, 1992). A sediment accumulation rate of 2.0 mm year1 was estimated for the two study sites. As mentioned above, sand content increased almost consistently from the w20 cm depth, to the top of both cores (Fig. 2). The OM profiles show opposing pattern in relation to the sand profiles. As shown in Fig. 4, the OM content in the PAA core decreases from the 16 cm depth to the surface. A similar pattern is seen in the OM profile of the GTA core, with the exception that the OM values decrease from the 8 cm depth. The chlorophyll-a and pheopigments (mg/g) values are generally higher towards the surface of both cores, reaching more consistent concentrations below the w10 cm depth (Table 1). A slightly lower pheo-pigment/chlorophyll-a ratio in the surface layers of both cores may be noted. Organic carbon burial rates, though generally higher in the Guaratuba Bay site, oscillate from 123 to 349 g m2 year1 along the time interval of this study (Table 1). 4. Discussion
6 3
210
PAA
GTA
4 4
Fig. 2. Sand (>63 mm) (dry wt %) profile distributed against date (year), considering the 2 mm year1 sediment accumulation rates.
As sea level rise may directly influences the lithostratigraphic changes in mangrove sediments, we will investigate these changes through OM and granulometric depositional trends, compare sediment accumulation rates to RSL rise, then discuss the implications of these results.
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
OM (%) 2003 1993 1983
year
1973 1963 1950 1930 1910 1890
13 9 9 10 12 15 16 16 16 17 19 20 20 21 20 15 14
20 22 22
PAA
GTA
24 25 25 27 29 29 27 27 22 26 24 24 23 24
Fig. 4. OM (dry wt %) profile distributed against date (year), considering the 2 mm year1 sediment accumulation rates.
Results of the stratigraphic analyses of the cores from both the study sites (located w50 km apart) indicate trends of net decreasing upcore in OM contents, in successfully younger sediments (Fig. 4). These trends are in contrast to the models developed widely on early (burial) diagenesis of OM (Berner, 1980; among several others) as enumerated in the following. Assuming that there is a steady-state depositional flux of OM, no temporal variations in lithology and the mangrove environment remains stable (especially the redox potential) a net decrease in OM contents will be the invariable norm with increasing core depth, as a consequence of increased early diagenesis accompanied by successively greater remineralization of OM downcore (Berner, 1980; Lynch et al., 1989; Cahoon and Lynch, 1997; Lallier-Verges et al., 1998; Chen and Twilley, 1999; Kristensen et al., 2000). Mangrove ecosystems have the capability of retaining the original high depositional flux of OM because the organic-rich sediments in this system are invariably anoxic and, therefore, relatively less prone to loss of OM by reminrealization (Kristensen et al., 2000; Twilley et al., 1986). The above trend has not prevailed at our study sites (Laranjeiras and Guaratuba Bays), during at least the past century. One depositional condition that apparently has impacted the expected (normal) profile of OM is a temporal change in the sediment granulometry. As illustrated in Fig. 2 there has been a net coarsening in lithology upcore in both GTA and PAA core sites, Table 1 Date (considering depth and a sediment accumulation rate of 2 mm year1) of chlorophyll-a and pheo-pigment contents (ug/g) as well as the organic carbon burial rates (OC g m2 year1) of both sediment cores in this study. year
PAA Clor-a/OC
2003 1998 1993 1988 1983 1978 1973 1968 1963 1958 1950 1940 1930 1920 1910 1900 1890
15.1 24.5 29.2 19.8 12.4 13.6 8.3 7.1 6.9 7.9 9.3 4.1 6.0 3.5 6.8 7.9 7.4
Pheo/ OC
Pheo/ Clor-a
OC burial
29.1 51.2 52.0 47.0 41.8 24.9 25.7 30.4 38.3 27.7 22.3 25.8 20.2 17.0 13.6 24.9 23.3
1.9 2.1 1.8 2.4 3.4 1.8 3.1 4.3 5.6 3.5 2.4 6.3 3.4 4.9 2.0 3.2 3.1
171 123 119 126 148 221 189 162 162 165 169 152 153 172 180 125 125
GTA Clor-a/OC 26.4 16.7 16.2 11.4 10.3 9.3 6.3 7.2 6.6 7.0 6.7 11.1 6.8 5.6 5.6 6.3 5.9
Pheo/ OC
Pheo/ Clor-a
OC burial
71.7 61.4 53.7 31.9 35.7 35.1 29.8 23.6 25.1 24.5 24.3 38.9 27.2 27.9 31.8 28.2 34.5
2.7 3.7 3.3 2.8 3.5 3.8 4.7 3.3 3.8 3.5 3.6 3.5 4.0 5.0 5.7 4.5 5.8
256 326 238 290 270 246 323 349 311 259 272 233 305 258 227 202 224
153
which matches with net decrease upcore in OM (Fig. 4). The 210Pbex profiles indicate no physical and/or bioturbation in the Guaratiba mangrove while the sediment layers appear to be mixed from the surface to the 3.5 cm depth in the Laranjeiras mangrove site. Below this surface layer the 210Pbex profiles indicate little, if any, physical and/or bioturbation. This lithostratigraphic changes demonstrates significant lateral variations recently in the environment of the study sites, with an overall substitution by an environment conducive to relatively greater sand deposition under more severe and open water tidal action (e.g., tidal flat)(Cohen et al., 2005; Ferreira et al., 2010). It is suggested that the net decrease in OM upcore is a combined result of dilution by coarser sediments and increased decomposition of OM in more aerated (less anoxic) ocean waters under successive environment. Similar temporal shifts in coastal environments accompanied by sediment characteristics have been noted elsewhere (Allison et al., 2003; Belperio et al., 2002; Heap et al., 2004; Kolker et al., 2010; Woodroffe et al., 1993). The chlorophyll-a and pheo-pigment contents in surficial sediments are often used to indicate the level of primary production of overlying waters or reflect the level of diagenesis of mangrove litter, algal mats and/or marine plankton deposited, among other factors (Misic et al., 2011; Holmer, 1999; Sun et al., 1991). The deposition of this material is evidenced by the higher values in the surface layers (>10 cm) of the study areas. Anoxic and thus favourable conditions for OM preservation, which are supported by the low pheopigments/chlorophyll-a ratios (Table 1) (Reuss et al., 2005), may be noted for both study areas. Both cores contain larger contents of the pigments and the degradation byproduct pheo-pigments in the surface layers (Table 1). However, some diagenesis is occurring in the upper sediments as shown by pheo-pigment concentrations being higher than chlorophyll concentrations. It is suggested that the significant decrease in the pigments in the lower half of the cores is likely to be attributed to a greater loss by remineralization of the labile organics at depth. Another possible reason for the decreased pigments at lower core depths could be a result of the bacteria-fungi-algae communities that live in the mangrove’s rhizosphere (Holguin et al., 2001) or higher dilution by mangrove vegetation debris in the past. The suggested progressive migration landward of the mangrove forests at the tectonically stable Laranjeiras and Guaratuba Bays could be a response to a local shoreline shift due to continued rise in RSL rise (Allison et al., 2003; Belperio et al., 2002; Heap et al., 2004; Woodroffe et al., 1993) as a consequence to climate change (IPCC, 2007). Such shifts have been not only documented in South Florida (Ross, 2000) but also in Northern Brazil (Cohen et al., 2005) using a similar type of sediment profiling as this study. If the RSL rise were to exceed sediment accumulation rate, the mangrove forest will tend to migrate landward or die off (Alongi, 2008). The southern coastline of Brazil, similar to many other regions of the world, may be specifically vulnerable to the relationship between sea level rise and mangrove forest sediment accumulation rates. This relation is particularly relative because if sediment accumulation rates do not overcompensate the sea level rise on coastlines constricted geologically or by urban development, mangrove forest will die off if alternative accommodation space is not available for the mangrove setback. The organic carbon burial rates in the two mangrove ecosystems of this study (Table 1) are estimated to be near global averages (w200 g m2 year1) (Chmura et al., 2003; Bouillon et al., 2008). Organic carbon burial rates are important because they are what are being considered, beside pools, in environmental policy and global organic carbon budges (Alongi, 2011; Bouillon et al., 2008). The mangrove forests in the study region, as well as in the entire South-Southeastern region of Brazil, are lined by the Serra do Mar coastal mountain range. As the base of the mountain range near the
154
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
study sites is approximately 8 m are slightly above sea level, there is a limit to the possible landward mangrove forest migration as a consequence of increasing sea level rise, particularly under the projected global warming scenario (IPCC, 2007). The IPCC (2007) scenario is of interest to mangrove ecosystems, as these systems are among the most carbon-rich forests in the tropics, of high productivity and OM burial (Alongi, 2011; Donato, 2011). Estimates on mangrove ecosystem carbon pools indicate considerable values, near 1000 Mg ha1, most of which is stored belowground (Donato, 2011). In Brazil alone there are more than one million hectares of mangrove forests (w1 Pg of organic carbon, using the Donato, 2011 estimates), much of which is near mountain range base as is the case in this study. This value is almost 10% of global average mangrove organic carbon pool. Therefore, it is not only important to note organic carbon pools but processes taking place in individual forests. The characterization of mangrove sediments may give insight to some of theses process. As depositional modifications may alter mangrove peat deposits, directly influencing the coastal ocean organic carbon budgets, the stability of mangrove systems should be addressed taking into account the relative sea level rise and accumulation rates in specific topographical regions of the world. 5. Conclusion Our conclusions can be summarized as follows: a) Sediment accumulation rates of near 2.0 mm year1, based on a 210Pbex CIC dating model, were estimated for the two study sites. b) Lithostratigraphic changes demonstrate significant lateral variations recently in the environment of the study sites, where sand deposition is replacing the mud (<63 mm) fraction. c) Results of the stratigraphic analyses of the cores from both the study sites indicate trends of net decreasing upcore in OM contents, in successfully younger sediments conducive to coastal areas were sediment accumulation rates are not keeping pace with climate associated sea level rise. d) Sea level rise, in turn, may directly influence the origin and quantity of OM deposited along mangrove sediments as related to forest migration in geographically constricted areas, likely playing a significant role in coastal ocean organic carbon cycles. Acknowledgements This work was funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil, Fulbright support to J.M.S. as well as CAPES and FAPERJ support (Grant E-26/101.952/ 2009) to C.J.S. We would like to acknowledge the Instituto de Radioproteção e Dosimetria (IRD) for supplying a certified radionuclide cocktail that was used for calibrating the gamma-ray detector. References Allison, M.A., et al., 2003. Stratigraphic evolution of the late Holocene Ganges Brahmaputra lower delta plain. Sedimentary Geology 155, 317e342. Alongi, D.M., 2008. Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuarine Coastal and Shelf Science 76, 1e13. Alongi, D.M., 2011. Carbon payments for mangrove conservation: ecosystem constraints and uncertainties of sequestration potential. Environmental Science & Policy 14, 462e470. Appleby, P.G., Oldfield, F., 1992. Application of lead-210 to sedimentation studies. In: Ivanovich, M., Harmon, S. (Eds.), Uranium Series Disequilibrium: Application to Earth, Marine and Environmental Science. Oxford Science Publications, pp. 731e783. Appleby, P.G., 1988. Pb-210 dating of lake sediments and ombrotrophic peats by gamma essay. Science of the Total Environment 68, 157e177.
Belperio, A.P., Harvey, N., Bourman, R.P., 2002. Spatial and temporal variability in the Holocene sea-level record of the South Australian coastline. Sedimentary Geology 150, 153e169. Berner, R.A., 1980. Early Diagenesis. Princeton Univ Press, Princeton, New Jersey. Bouillon, S., et al., 2008. Mangrove production and carbon sinks: a revision of global budget estimates. Global Biogeochemistry Cycles 22, 1e22. Cahoon, D.R., Lynch, C.J., 1997. Vertical accretion and shallow subsidence in a mangrove, forest of Southwestern Florida, U.S.A. Mangroves and Salt Marshes 1, 173e186. Chen, R., Twilley, R.R., 1999. A simulation model of organic matter and nutrient accumulation in mangrove wetland soils. Biogeochemistry 44, 93e118. Chmura, G.L., et al., 2003. Global carbon sequestration in tidal, saline wetland soils. Global Biogeochemistry Cycles 17, 1e12. Cohen, M.C.L., Behling, H., Lara, R.J., 2005. Amosonian Mangrove dynamics during the last millennium: the relative sea-level and Little Ice Age. Review of Palaeobotany & Palynology 136, 93e108. Cutshall, N.H., Larsen, I.L., Olsen, C.R., 1982. Direct analysis of 210Pb in sediment samples: self-absorption Corrections. Nuclear Instrument Methods 206, 309e312. Donato, D.C., et al., 2011. Mangroves among the most carbon-rich forests in the tropics. Nature Geoscience 4, 293e297. Ferreira, T.O., et al., 2010. Spatial patterns of soil attributes and components in a mangrove system in Southeast Brazil (São Paulo). Journal of Soils and Sediments 10, 995e1006. Gilman, E., Ellison, J., Coleman, R., 2007. Assessment of mangrove response to projected relative sea-level rise and recent historical reconstruction of shoreline position. Environmental Monitoring Assessment 124, 105e130. Hansen, J., et al., 2005. Earth’s energy imbalance: confirmation and implication. Science 308, 1431e1435. Heap, A.D., Bryce1,, S., Ryan, D.A., 2004. Facies evolution of Holocene estuaries and deltas: a large-sample statistical study from Australia. Sedimentary Geology 168, 1e17. Holguin, G., Vazquez, P., Bashan, Y., 2001. The role of sediment microorganisms in the productivity, conservation, and rehabilitation of mangrove ecosystems: an overview. Journal of Coastal Conservation 15, 629e638. Holmer, M., 1999. The Effect of Oxygen depletion on anaerobic organic matter degradation in marine sediments. Estuarine Coastal and Shelf Science 48, 383e390. IPCC, 2007. Climate Change 2007: The Physical Science Basis, Intergovernmental Panel on Climate Change. Paris. Kim, J., et al., 2005. Impacts of rapid sea-level rise on mangrove deposit erosion: application of taraxerol and Rhizophora records. Journal of Quaternary Science 20 (3), 221e225. Kolker, A.S., Kirwan, M., Goodbred, S.L., Cochran, J.K., 2010. Global climate changes recorded in coastal wetland sediments: empirical observations lined to theoretical predictions. Geophysical Research Letters 37, L14706, doi:10.1029/ 2010GL043874. Krauss, K.W., From, A.S., Doyle, T.W., Doyle, T.J., Barry, M.J., 2011. Sea-level rise and, landscape change influence mangrove encroachment onto marsh in the Ten Thousand Islands region of Florida, USA. Journal of Coastal Conservation 15, 629e638. Kristensen, E., et al., 2000. Carbon and nitrogen mineralization in sediments of the Bangrong, mangrove area, Phuket, Thailand. Aquatic Microbiology and Ecology 22, 199e213. Lallier-Verges, E., et al., 1998. Relationships between environmental conditions and the diagenetic evolution of organic matter derived from higher plants in a modern mangrove swamp system (Guadeloupe, French West Indies). Organic Geochemistry 29 (5e7), 1663e1686. Lopez-Medell, X., et al., 2011. Oceanographic anomalies and sea-level rise drive mangroves inland in the Pacific coast of Mexico. Journal of Vegetation Science 22, 143e151. Lorenzen, C.J., 1967. Determination of chlorophyll and pheo-pigments: spectrophotometric equations. Limnology and Oceanography 12, 343e346. Lynch, C.J., et al., 1989. Recent accretion in mangrove ecosystems based on Cs137and Pb-210. Estuaries 12 (4), 284e299. Meehl, G.A., et al., 2005. How much more global warming and sea level rise? Science 307, 1769e1772. Misic, C., et al., 2011. Organic matter features, degradation and remineralisation at two coastal sites in the Ligurian Sea (NW Mediterranean) differently influenced by anthropogenic forcing. Marine Environmental Research 72, 67e74. Perry, C.T., Berkeley, A., Smithers, S.G., 2008. Microfaciescharacterisitcs of a tropical, mangrove-fringed shoreline, Cleveland Bay, Queensalnd, Australia: sedimentary and taphonomic controls on mangrove facies development. Journal Sedimentary Research 78, 77e97. Pikey, O.H., Cooper, J.A.G., 2004. Society and sea level rise. Science 303, 1781e1782. Plante-Cuny, M.R., 1978. Pigments photosynthetiques et production primaire des fondsmeubles meritiquesd’une region tropicale Trav. Doc., 1. ORSTOM, Madagascar, 359 pp. Ravichandran, et al., 1995. Geochronology of sediments in the Sabine-Neches estuary, Texas U.S.A. Chemical Geology 125, 291e306. Reuss, N., Conley, D.J., Bianchi, T.S., 2005. Preservation conditions and the use of sediment pigments as a tool for recent ecological reconstruction in four Northern European estuaries. Marine Chemistry 95, 283e302.
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155 Ross, M.S., et al., 2000. The Southeast saline Everglades Revisited: 50 years of coastal vegetation change. Journal of Vegetation Science 11, 101e112. Sanders, C.J., et al., 2008. Recent sediment accumulation in a mangrove forest and Its relevance to local sea level rise (Ilha Grande, Brazil). Journal of Coastal Research 24 (2), 533e536. Sanders, C.J., et al., 2010a. Organic carbon burial in a mangrove forest, margin and intertidal mud flat. Estuarine Coastal and Shelf Science 90, 168e172. Sanders, C.J., et al., 2010b. Mangrove forest sedimentation and its reference to sea level rise, Cananeia, Brazil. Environmental Earth Science 60, 1291e1301. Schumacher, B.A., 2002. Methods for the Determination of Total Organic Carbon (TOC) in Soils and Sediments. United States Environmental Protection Agency, Ecological Risk Assessment Support Center Office of Research and Development, pp. 1e25.
155
Sun, M., Aller, R., Lee, C., 1991. Early diagenesis of chlorophyll-a in Long Island Sound sediments: a measure of carbon flux and particle reworking. Journal of Marine Research 49, 379e401. Twilley, R.R., Lugo, A.E., Patterson-Zucca, C., 1986. Litter production and turnover in basin mangrove forests in southwest Florida. Ecology 67 (3), 670e683. Woodroffe, C.D., Mulrennan, M.E., Chappell, J., 1993. Estuarine infill and coastal progradation, southern van Diemen Gulf, northern Australia. Sedimentary Geology 83, 257e275. Woodroffe, C.D., 1990. The impact of sea level rise on mangrove shorelines. Progress in Physical Geography 14, 483e520. Young, B.M., Harveya, L.E., 1996. A spatial analysis of the relationship between Mangrove (Avicennia marina var.australasica) Physiognomy and sediment accretion in the Hauraki Plains, New Zealand. Estuarine, Coastal and Shelf Science 42, 231e246.
Contents lists available at SciVerse ScienceDirect
Marine Environmental Research journal homepage: www.elsevier.com/locate/marenvrev
Short communication
Organic matter content and particle size modifications in mangrove sediments as responses to sea level rise Christian J. Sanders a, *, Joseph M. Smoak b, Mathew N. Waters c, Luciana M. Sanders d, Nilva Brandini a, Sambasiva R. Patchineelam a a
Universidade Federal de Fluminense (UFF), Departamento de Geoquímica, Niterói-RJ, Brazil University of South Florida (USF), Environmental Science, St. Petersburg, FL, USA Valdosta State University, Biology Department, Valdosta, GA, USA d Universidade Federal do Rio de Janeiro, Laboratório de Biogeoquímica, Rio de Janeiro - RJ, Brazil b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 July 2011 Received in revised form 26 November 2011 Accepted 8 February 2012
Mangroves sediments contain large reservoirs of organic material (OM) as mangrove ecosystems produce large quantities and rapidly burial OM. Sediment accumulation rates of approximately 2.0 mm year1, based on 210Pbex dating, were estimated at the margin of two well-developed mangrove forest in southern Brazil. Regional data point to a relative sea level (RSL) rise of up to w4.0 mm year1. This RSL rise in turn, may directly influence the origin and quantity of organic matter (OM) deposited along mangrove sediments. Lithostratigraphic changes show that sand deposition is replacing the mud (<63 mm) fraction and OM content is decreasing in successively younger sediments. Sediment accumulation in coastal areas that are not keeping pace with sea level rise is potentially conducive to the observed shifts in particle size and OM content. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Organic matter Deposition rate 210 Pb Dating Sedimentation rate Mangroves
1. Introduction The (sub)tropical coastal ocean shorelines of the world are widely lined with mangrove forests. Mangrove forest provide numerous ecosystem services as well as produce and retain large quantities of organic matter (OM) within their sediments and play an important role in coastal nutrient cycling (Perry et al., 2008; Sanders, 2010a: Ferreira et al., 2010). Also, mangroves efficiently trap fine sediments by slowing water movement to speeds conducive for particle deposition (Cahoon and Lynch, 1997; Young and Harveya, 1996). Mangrove sediments are influenced by many factors including daily changes in tidal energy, temperature, salinity and varying levels of anoxia (Alongi, 2008). The sedimentation processes in mangrove habitats are ultimately controlled by local hydrodynamics, including relative sea level (RSL) oscillations, tropical storms and sediment supply (Cahoon and Lynch, 1997; Lynch et al., 1989; Sanders, 2008; Alongi, 2008). The relationship between climate change and RSL fluctuations is an important issue globally (Hansen et al., 2005; Meehl et al., 2005).
* Corresponding author. Tel.: þ55 021 2629 2200. E-mail address: [email protected] (C.J. Sanders). 0141-1136/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.marenvres.2012.02.004
The Intergovernmental Panel on Climate Change (IPCC, 2007) has projected that the rate of global sea level rise, consequent to climate change, may increase up to 5.9 mm year1 by 2100. Regional variations in the RSL rates are found along the coastal regions of the world ocean (Pikey and Cooper, 2004) and have been shown to cause a direct response in mangrove forest sedimentation (Gilman et al., 2007). Kim et al. (2005) demonstrated mangrove displacement and lithostratigraphic profile modifications over approximately 20 kyr time period. Using stratigraphic sequences in mangrove peat deposits, Woodroffe (1990) concluded that mangroves may progressively move inland within decadal timescales. Indeed, many mangrove margins are experiencing considerable loss as a result of forest setback directly influenced by increased sea level rise (Lopez-Medell et al., 2011; Krauss et al., 2011). For example, in South Florida, mangrove forests have migrated inland by 1.5 km since the mid-1940s (Ross, 2000). Other studies have shown that mangrove sediment accretion is capable of keeping pace with sea level rise, not affecting the lithostratigraphic profiles (Lynch et al., 1989; Sanders, 2008, 2010b). The goal in this investigation is to examine sediment accumulation rates and to determine the time-interval changes in the ecological and depositional history (grain size, OM, chlorophylla and pheo-pigment) at Rhizophora spp. dominant margins of two
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
well-developed mangrove forests in southern Brazil. Here we show how RSL rise is causing a decrease in the OM deposition in mangrove forests where sediment accumulation rates are not keeping pace with RSL rise. While entire forests might shift landward, at these particular locations, migration is limited by geological constraints (i.e., mountain range), which is the case for the entire South-Southeastern coast of Brazil and therefore could ultimately result in collapse of these mangrove ecosystems. 2. Material and methods 2.1. Study site The mangrove forests of our study are located in Guaratuba Bay (GTA) and Laranjeiras Bay (PAA), southern Brazil (Fig. 1). The two bays are surrounded by well-developed mangrove forests, forming drainage basins off the Serra do Mar mountain range. The study sites of this work were selected based on their location near the latitudinal tolerance were mangrove thrive. Just to the south, these ecosystems are replaced by salt marshes in similar coastal settings. This location is ideal to identify impacts of climate change on mangrove stability. Sediment cores were collected from sites that appeared spatially homogenous without any abrupt transitions or indications of perturbations and therefore representative of the system. Time-series sea levels, as indicated by a nearby tide gauge in the region, show a RSL of w4.0 mm year1 during the past w50 years, while sedimentation accumulation rates in a stationary mangrove forest north of these sites indicate a RSL rise of over 2.5 mm year1
151
during the 20th century (Sanders, 2010b). As in many regions of the world, data to determine RSL trends along the Brazil coast are limited. This is because, unlike many countries in the northern hemisphere, tide gauges in South America have not been utilized consistently enough to determine confident RSL trends; therefore the RSL rates used for comparative reasons in this study are limited. The Laranjeiras Bay estuary is adjacent to Paranaguá Bay, home to one of Brazil’s busiest ports, whereas the estuary of Guaratuba Bay is relatively small with little anthropogenic activity. Although Laranjeiras Bay is near Paranaguá Bay (Fig. 1), this study site is located in an environmentally protected area, w30 km from the port. The specific mangrove study sites were chosen in regions unaffected by direct urban development. The estuaries are situated about w90 km SoutheSoutheast of the industrialized city Curitiba, which is 950 m above sea level. The annual average temperature and precipitation in the region is w21 C and w22 cm respectively. 2.2. Sediment sampling and analyses A sediment core from each site was collected in 2005 by inserting a 7 cm diameter acrylic tube into the substrate during low tide. The sediment was extruded out of the tube and sectioned at 1-cm intervals from the core top to 10 cm mark and subsequently at 2cm intervals to the bottom of the core. Organic matter was determined through loss of ignition (LOI), at 550 C for 2 h. Organic carbon values were estimated from loss on ignition using the conversion quotient of 1.724 (Schumacher, 2002). From the original wet section, an aliquot was taken for determining dry bulk density (DBD), which was estimated by taking into consideration the split’s
Fig. 1. Study area. a) Laranjeiras Bay and Guaranatuba Bay, Brazil; b) illustration of a mangrove margin where samples were taken.
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
a
6.0 5.5
LN(210Pbex)Bq/kg
porosity based on fractional water content and density of the interstitial water, OM and granulometry. Granulometric analyses were done using a CILAS 1064 diffraction laser unit. The chlorophylla and pheo-pigments were analyzed according to Plante-Cuny (1978), using a mod. Shimadzu UV-1601 optic density absorption spectrometer. Pigment content calculation was after Lorenzen (1967) and expressed as mg per g of organic carbon. For radionuclide analyses, the remaining sediment at each interval was sealed in 70 ml petri-dishes for at least three weeks, to establish secular equilibrium between 226Ra and 214Pb. Gamma-ray measurements for the radionuclide activities were achieved by using a semi-planar intrinsic germanium high purity coaxial detector with 40% efficiency, housed in a lead shield, coupled to a multichannel analyzer. Leade210 activity was determined by the direct measurement of 46.5 KeV gamma peak, while 226Ra activity was calculated based on estimates of 214Pb at 351.9 KeV peak (Appleby, 1988). Activities were calculated by multiplying the counts per minute for each radionuclide minus background counts, by a factor that includes the gamma-ray intensity and detector efficiency. This factor was determined from standard calibrations using an efficiency curve obtained by measuring and analyzing a certified standard cocktail of radionuclides; this standard was certified by IRD e (Instituto de Radioproteção e Dosimetria), certified no C/87/A00. The excess 210 Pb (210Pbex) activity was estimated by subtracting the 226Ra from the total 210Pb activity. Each sample was counted for 86,000 s in identical geometrical cylinders. Self-absorption correction was calculated following Cutshall et al. (1982). The sedimentation rates were obtained through the Constant Initial Concentration (CIC) dating method outlined in Appleby and Oldfield (1992).
5.0 4.5 4.0
GTA
3.5 3.0 2.5 0
3
6
9
12
15
18
16
20
24
Depth (cm)
b
5.5 5.0 4.5
LN(210Pbex)Bq/kg
152
4.0 3.5 3.0
PAA 2.5 2.0 1.5 1.0 0
4
8
12
Depth (cm) 210
Fig. 3. Mud - normalized Pbex distribution for sediment cores; (a) GTA and (b) PAA, distributed against depth (cm).
3. Results A net decrease in the 210Pbex activity, down the core depth, was recorded in both profiles (Fig. 2). Sand (>63 mm) content could be an influencing factor in the 210Pbex distribution due to the granulometric thickening towards the top of both cores (Fig. 2). Thus, the 210Pbex was normalized to the contents of the mud fraction. This was done assuming that sand has little or no 210Pbex activity (Ravichandran, 1995). After the correction, in each of the cores the net log profile of 210Pbex was almost linear extending from below the apparently turbated surface layers to the core depth where the parent-supported 210Pb activity was at background level (Fig. 3a, b). The 210Pbex counting errors ranged between 8 and 16%, increasing towards the bottom of the cores. As the net log linear profiles of the
Sand (%) 57
2003 1993 1983
year
1973 1963 1950
37 37 42 38
1930 1910 1890
20
70 67 59 54 61 54 49 55 49 49
26 25
22 34 24 21 18 19 21 18 13 9 7 5
Pbex were linear, the constant initial concentration (CIC) dating method as adopted in this study was justified. The statistics related to the cores’ 210Pbex distribution are as follows; Core PAA (y ¼ 0.16x þ 5.54) (r2 ¼ 0.89; n ¼ 14; p < 0.05) from the 3.5 to the 23 cm interval; Core GTA (y ¼ 0.15x þ 5.65) (r2 ¼ 0.89; n ¼ 14; p < 0.05) from the surface to the 17 cm mark of the cores. The CIC dating method implies that the sediment accumulation rate has been consistent during the time span encompassed in our study (Appleby and Oldfield, 1992). A sediment accumulation rate of 2.0 mm year1 was estimated for the two study sites. As mentioned above, sand content increased almost consistently from the w20 cm depth, to the top of both cores (Fig. 2). The OM profiles show opposing pattern in relation to the sand profiles. As shown in Fig. 4, the OM content in the PAA core decreases from the 16 cm depth to the surface. A similar pattern is seen in the OM profile of the GTA core, with the exception that the OM values decrease from the 8 cm depth. The chlorophyll-a and pheopigments (mg/g) values are generally higher towards the surface of both cores, reaching more consistent concentrations below the w10 cm depth (Table 1). A slightly lower pheo-pigment/chlorophyll-a ratio in the surface layers of both cores may be noted. Organic carbon burial rates, though generally higher in the Guaratuba Bay site, oscillate from 123 to 349 g m2 year1 along the time interval of this study (Table 1). 4. Discussion
6 3
210
PAA
GTA
4 4
Fig. 2. Sand (>63 mm) (dry wt %) profile distributed against date (year), considering the 2 mm year1 sediment accumulation rates.
As sea level rise may directly influences the lithostratigraphic changes in mangrove sediments, we will investigate these changes through OM and granulometric depositional trends, compare sediment accumulation rates to RSL rise, then discuss the implications of these results.
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
OM (%) 2003 1993 1983
year
1973 1963 1950 1930 1910 1890
13 9 9 10 12 15 16 16 16 17 19 20 20 21 20 15 14
20 22 22
PAA
GTA
24 25 25 27 29 29 27 27 22 26 24 24 23 24
Fig. 4. OM (dry wt %) profile distributed against date (year), considering the 2 mm year1 sediment accumulation rates.
Results of the stratigraphic analyses of the cores from both the study sites (located w50 km apart) indicate trends of net decreasing upcore in OM contents, in successfully younger sediments (Fig. 4). These trends are in contrast to the models developed widely on early (burial) diagenesis of OM (Berner, 1980; among several others) as enumerated in the following. Assuming that there is a steady-state depositional flux of OM, no temporal variations in lithology and the mangrove environment remains stable (especially the redox potential) a net decrease in OM contents will be the invariable norm with increasing core depth, as a consequence of increased early diagenesis accompanied by successively greater remineralization of OM downcore (Berner, 1980; Lynch et al., 1989; Cahoon and Lynch, 1997; Lallier-Verges et al., 1998; Chen and Twilley, 1999; Kristensen et al., 2000). Mangrove ecosystems have the capability of retaining the original high depositional flux of OM because the organic-rich sediments in this system are invariably anoxic and, therefore, relatively less prone to loss of OM by reminrealization (Kristensen et al., 2000; Twilley et al., 1986). The above trend has not prevailed at our study sites (Laranjeiras and Guaratuba Bays), during at least the past century. One depositional condition that apparently has impacted the expected (normal) profile of OM is a temporal change in the sediment granulometry. As illustrated in Fig. 2 there has been a net coarsening in lithology upcore in both GTA and PAA core sites, Table 1 Date (considering depth and a sediment accumulation rate of 2 mm year1) of chlorophyll-a and pheo-pigment contents (ug/g) as well as the organic carbon burial rates (OC g m2 year1) of both sediment cores in this study. year
PAA Clor-a/OC
2003 1998 1993 1988 1983 1978 1973 1968 1963 1958 1950 1940 1930 1920 1910 1900 1890
15.1 24.5 29.2 19.8 12.4 13.6 8.3 7.1 6.9 7.9 9.3 4.1 6.0 3.5 6.8 7.9 7.4
Pheo/ OC
Pheo/ Clor-a
OC burial
29.1 51.2 52.0 47.0 41.8 24.9 25.7 30.4 38.3 27.7 22.3 25.8 20.2 17.0 13.6 24.9 23.3
1.9 2.1 1.8 2.4 3.4 1.8 3.1 4.3 5.6 3.5 2.4 6.3 3.4 4.9 2.0 3.2 3.1
171 123 119 126 148 221 189 162 162 165 169 152 153 172 180 125 125
GTA Clor-a/OC 26.4 16.7 16.2 11.4 10.3 9.3 6.3 7.2 6.6 7.0 6.7 11.1 6.8 5.6 5.6 6.3 5.9
Pheo/ OC
Pheo/ Clor-a
OC burial
71.7 61.4 53.7 31.9 35.7 35.1 29.8 23.6 25.1 24.5 24.3 38.9 27.2 27.9 31.8 28.2 34.5
2.7 3.7 3.3 2.8 3.5 3.8 4.7 3.3 3.8 3.5 3.6 3.5 4.0 5.0 5.7 4.5 5.8
256 326 238 290 270 246 323 349 311 259 272 233 305 258 227 202 224
153
which matches with net decrease upcore in OM (Fig. 4). The 210Pbex profiles indicate no physical and/or bioturbation in the Guaratiba mangrove while the sediment layers appear to be mixed from the surface to the 3.5 cm depth in the Laranjeiras mangrove site. Below this surface layer the 210Pbex profiles indicate little, if any, physical and/or bioturbation. This lithostratigraphic changes demonstrates significant lateral variations recently in the environment of the study sites, with an overall substitution by an environment conducive to relatively greater sand deposition under more severe and open water tidal action (e.g., tidal flat)(Cohen et al., 2005; Ferreira et al., 2010). It is suggested that the net decrease in OM upcore is a combined result of dilution by coarser sediments and increased decomposition of OM in more aerated (less anoxic) ocean waters under successive environment. Similar temporal shifts in coastal environments accompanied by sediment characteristics have been noted elsewhere (Allison et al., 2003; Belperio et al., 2002; Heap et al., 2004; Kolker et al., 2010; Woodroffe et al., 1993). The chlorophyll-a and pheo-pigment contents in surficial sediments are often used to indicate the level of primary production of overlying waters or reflect the level of diagenesis of mangrove litter, algal mats and/or marine plankton deposited, among other factors (Misic et al., 2011; Holmer, 1999; Sun et al., 1991). The deposition of this material is evidenced by the higher values in the surface layers (>10 cm) of the study areas. Anoxic and thus favourable conditions for OM preservation, which are supported by the low pheopigments/chlorophyll-a ratios (Table 1) (Reuss et al., 2005), may be noted for both study areas. Both cores contain larger contents of the pigments and the degradation byproduct pheo-pigments in the surface layers (Table 1). However, some diagenesis is occurring in the upper sediments as shown by pheo-pigment concentrations being higher than chlorophyll concentrations. It is suggested that the significant decrease in the pigments in the lower half of the cores is likely to be attributed to a greater loss by remineralization of the labile organics at depth. Another possible reason for the decreased pigments at lower core depths could be a result of the bacteria-fungi-algae communities that live in the mangrove’s rhizosphere (Holguin et al., 2001) or higher dilution by mangrove vegetation debris in the past. The suggested progressive migration landward of the mangrove forests at the tectonically stable Laranjeiras and Guaratuba Bays could be a response to a local shoreline shift due to continued rise in RSL rise (Allison et al., 2003; Belperio et al., 2002; Heap et al., 2004; Woodroffe et al., 1993) as a consequence to climate change (IPCC, 2007). Such shifts have been not only documented in South Florida (Ross, 2000) but also in Northern Brazil (Cohen et al., 2005) using a similar type of sediment profiling as this study. If the RSL rise were to exceed sediment accumulation rate, the mangrove forest will tend to migrate landward or die off (Alongi, 2008). The southern coastline of Brazil, similar to many other regions of the world, may be specifically vulnerable to the relationship between sea level rise and mangrove forest sediment accumulation rates. This relation is particularly relative because if sediment accumulation rates do not overcompensate the sea level rise on coastlines constricted geologically or by urban development, mangrove forest will die off if alternative accommodation space is not available for the mangrove setback. The organic carbon burial rates in the two mangrove ecosystems of this study (Table 1) are estimated to be near global averages (w200 g m2 year1) (Chmura et al., 2003; Bouillon et al., 2008). Organic carbon burial rates are important because they are what are being considered, beside pools, in environmental policy and global organic carbon budges (Alongi, 2011; Bouillon et al., 2008). The mangrove forests in the study region, as well as in the entire South-Southeastern region of Brazil, are lined by the Serra do Mar coastal mountain range. As the base of the mountain range near the
154
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155
study sites is approximately 8 m are slightly above sea level, there is a limit to the possible landward mangrove forest migration as a consequence of increasing sea level rise, particularly under the projected global warming scenario (IPCC, 2007). The IPCC (2007) scenario is of interest to mangrove ecosystems, as these systems are among the most carbon-rich forests in the tropics, of high productivity and OM burial (Alongi, 2011; Donato, 2011). Estimates on mangrove ecosystem carbon pools indicate considerable values, near 1000 Mg ha1, most of which is stored belowground (Donato, 2011). In Brazil alone there are more than one million hectares of mangrove forests (w1 Pg of organic carbon, using the Donato, 2011 estimates), much of which is near mountain range base as is the case in this study. This value is almost 10% of global average mangrove organic carbon pool. Therefore, it is not only important to note organic carbon pools but processes taking place in individual forests. The characterization of mangrove sediments may give insight to some of theses process. As depositional modifications may alter mangrove peat deposits, directly influencing the coastal ocean organic carbon budgets, the stability of mangrove systems should be addressed taking into account the relative sea level rise and accumulation rates in specific topographical regions of the world. 5. Conclusion Our conclusions can be summarized as follows: a) Sediment accumulation rates of near 2.0 mm year1, based on a 210Pbex CIC dating model, were estimated for the two study sites. b) Lithostratigraphic changes demonstrate significant lateral variations recently in the environment of the study sites, where sand deposition is replacing the mud (<63 mm) fraction. c) Results of the stratigraphic analyses of the cores from both the study sites indicate trends of net decreasing upcore in OM contents, in successfully younger sediments conducive to coastal areas were sediment accumulation rates are not keeping pace with climate associated sea level rise. d) Sea level rise, in turn, may directly influence the origin and quantity of OM deposited along mangrove sediments as related to forest migration in geographically constricted areas, likely playing a significant role in coastal ocean organic carbon cycles. Acknowledgements This work was funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil, Fulbright support to J.M.S. as well as CAPES and FAPERJ support (Grant E-26/101.952/ 2009) to C.J.S. We would like to acknowledge the Instituto de Radioproteção e Dosimetria (IRD) for supplying a certified radionuclide cocktail that was used for calibrating the gamma-ray detector. References Allison, M.A., et al., 2003. Stratigraphic evolution of the late Holocene Ganges Brahmaputra lower delta plain. Sedimentary Geology 155, 317e342. Alongi, D.M., 2008. Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuarine Coastal and Shelf Science 76, 1e13. Alongi, D.M., 2011. Carbon payments for mangrove conservation: ecosystem constraints and uncertainties of sequestration potential. Environmental Science & Policy 14, 462e470. Appleby, P.G., Oldfield, F., 1992. Application of lead-210 to sedimentation studies. In: Ivanovich, M., Harmon, S. (Eds.), Uranium Series Disequilibrium: Application to Earth, Marine and Environmental Science. Oxford Science Publications, pp. 731e783. Appleby, P.G., 1988. Pb-210 dating of lake sediments and ombrotrophic peats by gamma essay. Science of the Total Environment 68, 157e177.
Belperio, A.P., Harvey, N., Bourman, R.P., 2002. Spatial and temporal variability in the Holocene sea-level record of the South Australian coastline. Sedimentary Geology 150, 153e169. Berner, R.A., 1980. Early Diagenesis. Princeton Univ Press, Princeton, New Jersey. Bouillon, S., et al., 2008. Mangrove production and carbon sinks: a revision of global budget estimates. Global Biogeochemistry Cycles 22, 1e22. Cahoon, D.R., Lynch, C.J., 1997. Vertical accretion and shallow subsidence in a mangrove, forest of Southwestern Florida, U.S.A. Mangroves and Salt Marshes 1, 173e186. Chen, R., Twilley, R.R., 1999. A simulation model of organic matter and nutrient accumulation in mangrove wetland soils. Biogeochemistry 44, 93e118. Chmura, G.L., et al., 2003. Global carbon sequestration in tidal, saline wetland soils. Global Biogeochemistry Cycles 17, 1e12. Cohen, M.C.L., Behling, H., Lara, R.J., 2005. Amosonian Mangrove dynamics during the last millennium: the relative sea-level and Little Ice Age. Review of Palaeobotany & Palynology 136, 93e108. Cutshall, N.H., Larsen, I.L., Olsen, C.R., 1982. Direct analysis of 210Pb in sediment samples: self-absorption Corrections. Nuclear Instrument Methods 206, 309e312. Donato, D.C., et al., 2011. Mangroves among the most carbon-rich forests in the tropics. Nature Geoscience 4, 293e297. Ferreira, T.O., et al., 2010. Spatial patterns of soil attributes and components in a mangrove system in Southeast Brazil (São Paulo). Journal of Soils and Sediments 10, 995e1006. Gilman, E., Ellison, J., Coleman, R., 2007. Assessment of mangrove response to projected relative sea-level rise and recent historical reconstruction of shoreline position. Environmental Monitoring Assessment 124, 105e130. Hansen, J., et al., 2005. Earth’s energy imbalance: confirmation and implication. Science 308, 1431e1435. Heap, A.D., Bryce1,, S., Ryan, D.A., 2004. Facies evolution of Holocene estuaries and deltas: a large-sample statistical study from Australia. Sedimentary Geology 168, 1e17. Holguin, G., Vazquez, P., Bashan, Y., 2001. The role of sediment microorganisms in the productivity, conservation, and rehabilitation of mangrove ecosystems: an overview. Journal of Coastal Conservation 15, 629e638. Holmer, M., 1999. The Effect of Oxygen depletion on anaerobic organic matter degradation in marine sediments. Estuarine Coastal and Shelf Science 48, 383e390. IPCC, 2007. Climate Change 2007: The Physical Science Basis, Intergovernmental Panel on Climate Change. Paris. Kim, J., et al., 2005. Impacts of rapid sea-level rise on mangrove deposit erosion: application of taraxerol and Rhizophora records. Journal of Quaternary Science 20 (3), 221e225. Kolker, A.S., Kirwan, M., Goodbred, S.L., Cochran, J.K., 2010. Global climate changes recorded in coastal wetland sediments: empirical observations lined to theoretical predictions. Geophysical Research Letters 37, L14706, doi:10.1029/ 2010GL043874. Krauss, K.W., From, A.S., Doyle, T.W., Doyle, T.J., Barry, M.J., 2011. Sea-level rise and, landscape change influence mangrove encroachment onto marsh in the Ten Thousand Islands region of Florida, USA. Journal of Coastal Conservation 15, 629e638. Kristensen, E., et al., 2000. Carbon and nitrogen mineralization in sediments of the Bangrong, mangrove area, Phuket, Thailand. Aquatic Microbiology and Ecology 22, 199e213. Lallier-Verges, E., et al., 1998. Relationships between environmental conditions and the diagenetic evolution of organic matter derived from higher plants in a modern mangrove swamp system (Guadeloupe, French West Indies). Organic Geochemistry 29 (5e7), 1663e1686. Lopez-Medell, X., et al., 2011. Oceanographic anomalies and sea-level rise drive mangroves inland in the Pacific coast of Mexico. Journal of Vegetation Science 22, 143e151. Lorenzen, C.J., 1967. Determination of chlorophyll and pheo-pigments: spectrophotometric equations. Limnology and Oceanography 12, 343e346. Lynch, C.J., et al., 1989. Recent accretion in mangrove ecosystems based on Cs137and Pb-210. Estuaries 12 (4), 284e299. Meehl, G.A., et al., 2005. How much more global warming and sea level rise? Science 307, 1769e1772. Misic, C., et al., 2011. Organic matter features, degradation and remineralisation at two coastal sites in the Ligurian Sea (NW Mediterranean) differently influenced by anthropogenic forcing. Marine Environmental Research 72, 67e74. Perry, C.T., Berkeley, A., Smithers, S.G., 2008. Microfaciescharacterisitcs of a tropical, mangrove-fringed shoreline, Cleveland Bay, Queensalnd, Australia: sedimentary and taphonomic controls on mangrove facies development. Journal Sedimentary Research 78, 77e97. Pikey, O.H., Cooper, J.A.G., 2004. Society and sea level rise. Science 303, 1781e1782. Plante-Cuny, M.R., 1978. Pigments photosynthetiques et production primaire des fondsmeubles meritiquesd’une region tropicale Trav. Doc., 1. ORSTOM, Madagascar, 359 pp. Ravichandran, et al., 1995. Geochronology of sediments in the Sabine-Neches estuary, Texas U.S.A. Chemical Geology 125, 291e306. Reuss, N., Conley, D.J., Bianchi, T.S., 2005. Preservation conditions and the use of sediment pigments as a tool for recent ecological reconstruction in four Northern European estuaries. Marine Chemistry 95, 283e302.
C.J. Sanders et al. / Marine Environmental Research 77 (2012) 150e155 Ross, M.S., et al., 2000. The Southeast saline Everglades Revisited: 50 years of coastal vegetation change. Journal of Vegetation Science 11, 101e112. Sanders, C.J., et al., 2008. Recent sediment accumulation in a mangrove forest and Its relevance to local sea level rise (Ilha Grande, Brazil). Journal of Coastal Research 24 (2), 533e536. Sanders, C.J., et al., 2010a. Organic carbon burial in a mangrove forest, margin and intertidal mud flat. Estuarine Coastal and Shelf Science 90, 168e172. Sanders, C.J., et al., 2010b. Mangrove forest sedimentation and its reference to sea level rise, Cananeia, Brazil. Environmental Earth Science 60, 1291e1301. Schumacher, B.A., 2002. Methods for the Determination of Total Organic Carbon (TOC) in Soils and Sediments. United States Environmental Protection Agency, Ecological Risk Assessment Support Center Office of Research and Development, pp. 1e25.
155
Sun, M., Aller, R., Lee, C., 1991. Early diagenesis of chlorophyll-a in Long Island Sound sediments: a measure of carbon flux and particle reworking. Journal of Marine Research 49, 379e401. Twilley, R.R., Lugo, A.E., Patterson-Zucca, C., 1986. Litter production and turnover in basin mangrove forests in southwest Florida. Ecology 67 (3), 670e683. Woodroffe, C.D., Mulrennan, M.E., Chappell, J., 1993. Estuarine infill and coastal progradation, southern van Diemen Gulf, northern Australia. Sedimentary Geology 83, 257e275. Woodroffe, C.D., 1990. The impact of sea level rise on mangrove shorelines. Progress in Physical Geography 14, 483e520. Young, B.M., Harveya, L.E., 1996. A spatial analysis of the relationship between Mangrove (Avicennia marina var.australasica) Physiognomy and sediment accretion in the Hauraki Plains, New Zealand. Estuarine, Coastal and Shelf Science 42, 231e246.