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Temporal variability of carbon and nutrient burial, sediment accretion, and mass accumulation over the past century in a carbonate platform mangrove forest of the Florida Everglades

3. Results

> Results

3.1. Soil Bulk Density, Organic Matter, TOC, TN, TP, δ13C, and δ15N

The mean dry bulk density (DBD) for all intervals is 0.20 ± 0.09 (standard deviation) g cm-3 (Table 1). The mean OM percentage for all intervals is 52 ± 12. Overall, DBD is highest in the first 7 cm (0.29 ± 0.16 g cm-3) before becoming relatively uniform throughout intervals 8-40 cm (0.18 ± 0.04 g cm-3) (Figure 2). The pattern for OM percentage is opposite,with lower values in the top 7 cm (40 ± 15%) and higher, more uniform values over the remaining depths (55 ± 8%).

The mean total OC for the dated intervals of all six cores was 24 ± 6% or 44.9 ± 10.0 mg cm-3 (Table 1). The mean TN was 2.5 ± 0.9 mg cm-3, and TP was 0.21 ± 0.06 mg cm-3. Molar C:N for these bulk sediments was 21.2 ± 3.4, and N:P was 27.6 ± 8.5.

The mean δ13C was 26.4 ± 0.9‰ (Table 1). Variability was highest in the three upper intervals where the standard deviations were 2.5, 1.7, and 1.7 compared to a mean standard deviation of 0.6 ± 0.2 for the 3-30 cm depth intervals. The mean δ15N was 3.1 ± 0.5‰ (Table 1). Standard deviations were relatively similar throughout all depths (0.4 ± 0.2‰). However, core SH3-1 had substantial variability with five intervals interspersed throughout the core that had δ15N values greater than 4.0‰ (Figure 2).

Table 1. Mean and Standard Deviation (SD) of Soil Characteristics From Dated Intervals (~ ≤ 100 yrs)a
  Bulk Density
(g cm-3)
Organic Matter
(%)
Organic C
(%)
Organic C
(mg cm-3)
Total N
(mg cm-3)
Total P
(mg cm-3)
Atomic C:N Atomic N:P δ13C
(‰)
δ15N
(‰)
Core Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD
SH3-1 0.28 0.18 43% 13% 19% 6% 43.8 9.2 3.0 1.7 0.23 0.04 19.3 4.0 29.6 18.4 27.5 0.6 3.4 0.8
SH3-3 0.21 0.05 40% 6% 18% 2% 37.9 5.7 2.5 0.4 0.26 0.04 18.1 1.7 21.0 2.6 26.1 0.8 3.2 0.4
SH3-5 0.17 0.05 59% 9% 27% 3% 45.0 10.6 2.6 0.9 0.22 0.06 19.1 1.0 27.5 2.8 26.6 0.3 2.8 0.2
SH3-7 0.21 0.09 54% 12% 26% 7% 51.9 9.2 2.5 0.4 0.19 0.06 24.7 2.2 29.6 5.7 26.0 1.1 3.1 0.3
SH3-8 0.21 0.08 57% 12% 25% 6% 49.1 10.3 2.2 0.7 0.19 0.06 23.3 2.4 27.8 4.6 25.8 0.8 3.0 0.3
SH3-9 0.16 0.03 56% 12% 27% 4% 41.2 7.6 2.1 0.5 0.15 0.03 23.0 2.0 30.8 5.7 26.4 0.9 3.2 0.4
Site 0.20 0.09 52% 12% 24% 6% 44.9 10.0 2.5 0.9 0.21 0.06 21.2 3.4 27.6 8.5 26.4 0.9 3.1 0.5
aBold font values are the mean and standard deviation for all six cores


depth profiles of soil characteristics for dated intervals of the SH3-1 soil core
depth profiles of soil characteristics for dated intervals of the SH3-3 soil core
depth profiles of soil characteristics for dated intervals of the SH3-5 soil core
depth profiles of soil characteristics for dated intervals of the SH3-7 soil core
depth profiles of soil characteristics for dated intervals of the SH3-8 soil core
depth profiles of soil characteristics for dated intervals of the SH3-9 soil core
Figure 2. (a-f) Depth profiles of soil characteristics for dated intervals (≤100 years) of the six soil cores. Data gaps (ND) in cores SH3-5 and SH3-8 are due to lack of available sediment for analysis. [click on images abover for larger version]

3.2. Pb-210 Activities and CRS-Modeled Dates

The six cores show a net decrease in the activity of excess 210Pb (Table 2). Each also exhibits lower activities in the near-surface intervals that correspond to dilution from Hurricane Wilma's input of marine carbonate sediment that is low in 210Pb [Smoak et al., 2013]. There are several unusually high peaks in 210Pbex below 20 cm depth. This is likely due to advective mixing from crab burrows. The CRS dating model is relatively insensitive to mixing [Appleby and Oldfield, 1992], and these individual peaks do not alter results substantially. Core SH3-8 is an example of this; the largest of these peaks in 210Pbex activities occurs at 27-30 cm depth. If this peak is manually removed so that the points are linearly interpolated to activity levels between the points above and below the peak, the resulting changes to the measured rates are relatively minor. The total age of the profile increases by ~20 years; most of the date changes occur in the bottommost 210Pbex intervals where the ages are greater than 100 years. The 100 year rates remain largely the same, with accretion decreasing by 0.1 mm yr-1 and OC burial decreasing by 7 g m-2 yr-1.

Table 2. Excess 210Pb Activity (dpm g-1), CRS Model Age at Bottom of Each Interval (Years), and 1 Standard Age Error (Years) for Soil Coresa
  SH3-1 SH3-3 SH3-5 SH3-7 SH3-8 SH3-9
Depth Interval 210PbXS Age Age Error 210PbXS Age Age Error 210PbXS Age Age Error 210PbXS Age Age Error 210PbXS Age Age Error 210PbXS Age Age Error
0-1 1.9 2.6 0.5 3.7 1.5 0.7 4.7 1.0 0.6 4.4 2.8 1.5 4.1 2.0 0.6 4.4 2.1 0.6
1-2 1.0 3.5 0.5 4.4 4.4 0.7 5.0 2.2 0.6 4.1 5.3 1.6 3.8 4.1 0.6 4.3 3.7 0.6
2-3 1.0 4.6 0.6 3.9 6.9 0.7 5.2 3.4 0.6 4.3 7.4 1.6 3.9 6.4 0.6 7.6 5.9 0.7
3-4 4.4 6.2 0.6 4.4 9.7 0.7 4.2 5.9 0.7 4.7 9.8 1.7 4.3 10.2 0.6 7.4 8.1 0.7
4-5 6.2 8.4 0.6 3.6 12.1 0.8 4.4 7.3 0.7 4.9 14.8 1.8 4.4 13.4 0.7 6.8 10.4 0.7
5-6 5.7 10.3 0.6 3.9 13.8 0.8 7.1 9.4 0.7 4.3 18.5 2.0 4.1 15.1 0.7 7.2 13.0 0.8
6-7 6.6 12.7 0.6 5.6 16.1 0.8 5.4 11.1 0.7 4.2 23.3 2.2 6.1 17.4 0.7 7.3 16.1 0.8
7-8 4.4 14.6 0.7 4.7 18.3 5.1 12.6 0.7 0.9 4.4 26.2 2.3 5.5 20.1 0.7 7.0 18.3 0.9
8-9 5.3 16.9 0.7 4.4 21.1 0.9 5.3 14.3 0.8 4.9 30.6 2.6 4.2 22.1 0.8 6.3 21.4 0.9
9-10 3.4 18.3 0.7 4.4 23.7 1.0 5.0 15.9 0.8 4.8 34.2 2.8 3.8 24.1 0.8 5.3 24.1 1.0
10-11 4.5 22.2 0.8 3.1 25.4 1.0 5.6 18.4 0.8 4.3 38.1 3.0 3.7 26.2 0.8 4.2 26.8 1.0
11-12       3.4 28.1 1.0 5.4 20.3 0.8 5.5 42.3 3.3 4.4 28.3 0.9 3.3 28.3 1.0
12-13 2.9 24.9 0.9 3.7 31.2 1.1 4.1 22.3 0.8 5.0 47.6 3.7 4.3 31.1 0.9 3.5 30.9 1.1
13-14       3.6 33.9 1.2 4.2 24.2 0.9 2.9 49.9 3.9 3.7 34.1 1.0 2.7 33.2 1.1
14-15 2.8 28.0 0.9 3.5 36.7 1.2 4.5 25.7 0.9 3.0 52.4 4.1 3.4 37.0 1.0 3.0 36.1 1.1
15-16       3.3 39.7 1.3 4.3 27.1 0.9 3.0 56.7 4.4 3.3 39.8 1.1 3.0 38.1 1.2
16-17 2.6 33.1 1.0 3.1 43.2 1.4 4.4 29.1 1.0 3.6 61.9 5.0 3.2 42.4 1.1 3.2 40.7 1.3
17-18       3.0 47.7 1.6 4.9 31.5 1.0 2.7 66.9 5.6 3.1 44.8 1.2 3.3 43.6 1.3
18-19 2.0 36.2 1.1 2.5 51.7 1.7 4.7 33.4 1.0 2.2 71.9 5.9 2.9 47.5 1.3 3.8 49.4 1.4
19-20       2.4 56.2 1.9 4.5 36.0 1.1 2.1 77.8 6.4 2.7 50.5 1.3 3.8 55.3 1.6
20-21 2.6 40.8 1.2 2.5 62.1 2.2 5.1 39.0 1.1 1.8 83.2 7.0 2.9 54.6 1.4 3.7 61.0 1.8
21-22       2.3 68.0 2.5 5.3 42.7 1.2 1.9 89.5 7.9 2.6 58.5 1.6 3.6 68.4 2.1
22-23 1.6 44.5 1.3 1.9 73.0 2.8 5.4 46.2 1.3 1.7 97.4 9.1 3.0 63.7 1.8 2.5 75.2 2.5
23-24       1.6 79.2 3.3 4.8 50.9 1.5 2.6 116.4 14.0 2.9 69.0 2.0 2.4 82.0 2.9
24-25 2.9 52.4 1.6 1.5 85.7 3.9 4.8 56.2 1.6 1.6 134.5 20.3 1.5 73.0 2.1 2.4 91.2 3.5
25-26       1.4 93.3 4.7 4.1 60.0 1.8 1.2 163.6 35.7 1.5 77.0 2.3 3.3 102.1 4.5
26-27 1.8 58.1 1.8 1.3 104.9 6.2 6.0 67.7 2.2 0.8     1.2 80.1 2.2 2.3 120.3 7.2
27-28       1.2 120.1 8.3 3.6 74.0 2.5       2.5 91.1 2.6 1.4 140.2 12.1
28-29 1.7 64.2 2.2 1.3 149.8 17.7 2.5 79.7 2.8       4.1 111.2 4.1 1.6 222.1 98.7
29-30       0.6 186.6 40.7 2.4 87.7 3.3       3.3 147.8 10.9 0.1    
30-31 1.5 70.8 2.5 0.2     2.1 94.4 3.8       0.9 173.8 20.1      
31-32             1.8 102.4 4.5       0.5          
32-33 2.7 85.1 3.9       1.4 110.0 5.3                  
33-34             1.5 120.9 6.8                  
34-35 2.1 106.9 7.1       1.3 134.8 9.4                  
35-36             1.4 157.4 16.6                  
36-37 1.5 152.3 27.5       1.0 218.0 75.8                  
37-38             0.2                      
38-39 0.6                                  
39-40                                    
aCore SH3-1 was sectioned in 1 cm intervals until a depth of 10 cm; intervals below that depth are 2 cm.

3.3. Calculated Rates

Although there are numerous periods of increase or decrease between intervals in individual cores, in general, the rate profiles show a net decline with depth (Figure 3 and Figure 4). As a result the integrated rates over 10, 50, and 100 years decrease in each core record. The 10 year average accumulation rate for IM is 1028 ± 568, decreasing to 301 ± 104 g m-2 yr-1 for the 100 year average (Table 3). There is a prominent peak of IM accumulation in the top three intervals of SH3-1 that requires a scale adjustment compared to that used for the other five cores. The 10 year average accumulation rate for OM is 462 ± 97 decreasing to 267 ± 46 g m-2 yr-1 for the 100 year average (Table 3). The 10 year average OC burial rate is 225 ± 61, and decreases to 123 ± 19 g m-2 yr-1 over the 100 year period. Variability for TN and TP accumulation rates shows the same general decline from the 10 year to 100 year averages, but both show extensive spatial variability at the 10 year level. The 10 year accumulation rate for TN is 13.8 ± 6.3 and 7.0 ± 1.4 g m-2 yr-1 for the 100 year period. The 10 year accumulation rate for TP is 0.58 ± 0.25 and 0.20 ± 0.08 g m-2 yr-1 for the 100 year period (Table 3).

The density-corrected accretion rate from all six cores is 4.8 ± 1.0 mm yr-1 at the 10 year period and 2.7 ± 0.4 mm yr-1 at the 100 year period (Table 3). Cores SH3-1 and SH3-5 show a marked increase in accretion in the near-surface intervals following hurricane Wilma, with rates that exceed 10 and 8 mm yr-1, respectively (Figure 3). If the contributions from Hurricane Wilma in the period from 2000 to 2010 are excluded, the 100 yr mean accretion rate decreases only slightly to 2.6 ± 0.5. Both rates match the tide gauge record of sea level rise (2.24 ± 0.16 mm yr-1) from Key West, FL (approximately 120 km south of the research site), over the same time period [Maul and Martin, 1993; NOAA National Ocean Service, 2014].

Table 3. Ten Year, 50 Year, and 100 Year Site Mean Rates
  Timescale Site Mean SD
Inorganic matter (g m-2 yr-1) 10 year 1028 568
50 year 471 177
100 year 301 104
Organic matter (g m-2 yr-1) 10 year 462 97
50 year 380 73
100 year 267 46
Organic carbon (g m-2 yr-1) 10 year 225 61
50 year 176 31
100 year 123 19
Total nitrogen (g m-2 yr-1) 10 year 13.8 6.3
50 year 10.0 2.5
100 year 7.0 1.4
Total phosphorous (g m-2 yr-1) 10 year 0.58 0.25
50 year 0.35 0.15
100 year 0.20 0.08

Accretion (mm yr-1)

10 year 4.8 1.0
50 year 3.7 0.7
100 year 2.7 0.4


profiles of inorganic and organic matter accumulation, organic carbon burial, and accretion rates for each of the six soil cores
Figure 3. Profiles of inorganic and organic matter accumulation, OC burial, and accretion rates for each of the six soil cores. Note that the scales for SH3-1 IM and OM accumulation and accretion have been adjusted to account for the larger surface rates in that core [Smoak et al., 2013]. [larger image]


bar graphs showing site mean sediment accumulation rates aggregated by decade
Figure 4. Site mean sediment accumulation rates aggregated by decade. Bars represent 95% confidence interval of the six core mean rates for each decade. [larger image]

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