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Sediment accretion and organic carbon burial relative to sea-level rise and storm events in two mangrove forests in Everglades National Park

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4. Results

Both cores examined in this study have a typical exponential decrease in specific excess 210Pb activity moving down core with the exception of the low activity in the near-surface (Fig. 2). Over time the mass accumulation rates have varied considerably from the overall average, a notable example being a substantial increase in the 2000 to 2009 period for both cores (Fig. 3). The mass accumulation rate for this period is 4.4 times greater than the 1924-2000 rate for SH3, and 3.3 times greater than the 1926-2000 rate for SH4. This period of enhanced accumulation increased the long-term mass accumulation rates from 677 g m-2 yr-1 to 903 g m-2 yr-1 for SH3, and 447 to 545 for SH4 (Table 1). The high accumulation rates in the upper sections of each core are accompanied by relatively low organic carbon content (Fig. 4) indicating anomalous sediment input corresponding to the noted near-surface low in specific excess 210Pb.

Table 1. Mass accumulation rate (MAR), sediment accretion rate, and OC accumulation rate for given time interval.
Period (year)
MAR (g m-2 yr-1)
Accretion (mm yr-1)
Organic C (g m-2 yr-1)
SH3
1924 to 2009
903
3.6
151
2000 to 2009
2966
5.9
260
1924 to 2000
677
3.3
139
SH4
1926 to 2009
545
2.5
168
2000 to 2009
1457
6.5
393
1926 to 2000
447
2.1
144

The accretion rates for each interval (Fig. 5) are similar to mass accumulation rate trends. The average accretion rates for three distinct time periods are shown in Table 1. The 2000-2009 period accretion rate is 1.8 times greater than the 1924-2000 rate for SH3 and 3.1 times greater than the 1926-2000 rate for SH4.

The OC burial rates are both higher and strikingly different for each core during the 2000-2009 period (260 g m-2 yr-1 at SH3 and 393 g m-2 yr-1 at SH4). In contrast to the surface level rate differences between the two cores, overall OC burial rates were similar (151 g m-2 yr-1 at SH3 and 168 g m-2 yr-1 at SH4).

plot showing 210-lead activity moving down core for Shark 3 and Shark 4 mangrove cores
graphs showing mass accumulation rates over time for Shark 3 and Shark 4 mangrove cores
Fig. 2. (left) Excess 210Pb vs. depth profile for SH3 and SH4. [larger version]
Fig. 3. (right) Mass accumulation rate vs. age profile for SH3 and SH4. [larger version]


graphs showing organic carbon content over time for Shark 3 and Shark 4 mangrove cores
graphs showing sediment accretion rates over time for Shark 3 and Shark 4 mangrove cores
Fig. 4. (left) Percentage organic matter vs. age profile for SH3 and SH4. [larger version]
Fig. 5. (right) Sediment accretion rate vs. age profile for SH3 and SH4. [larger version]

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Last updated: 29 October, 2013 @ 12:52 PM (KP)