dennis

RATES OF RELATIVE SEA LEVEL RISE

AND CLIMATE CHANGE

OVER THE LAST MILLENNIUM:

A RECORD FROM

DENNIS CREEK, NJ

ELLEN THOMAS

and

JOOP VAREKAMP

Department of E & ES

Wesleyan University

Middletown, CT

MAIN POINTS:

The average rate of relative sea level rise in the Dennis Creek (NJ) salt marshes over the last 1200 years was ~ 2.3 mm/a; it accelerated to ~ 5.7 mm/a between 1600 and 1700 AD, and accelerated further to ~ 8.0 mm/a at about 1800 AD.

During the Mediaeval Warming relative sea level rise was not accelerated, but the increase in the 17^th century was about coeval with the end of the Little Ice Age.

Tidal marsh surrounding Dennis Creek:

Where is this marsh? In northwestern Cape May County, NJ, in the 'elbow' of the peninsula bounded on the east by the Atlantic Ocean, on the west by Delaware Bay, (39^o07' - 39^o12'N, 74^o49' - 74^o55'W).

On the west, the marsh borders Delaware Bay, in all other directions the marsh grades laterally into upland vegetation; along the north and east of the marsh are cedar swamps. Colored letters indicate locations of cores studied. Data will be shown for cores DCA, DCD/DCL and DCE.

History of the marsh:

After the last deglaciation, the region started to subside rapidly because of 'forebulge collapse', and the Dennis Creek marshes yield evidence for fluctuations in the rate of RSLR over the last 4000 yrs (Meyerson, 1978, Marine Geology, 12: 335-357). The high accretion rates provide the opportunity of obtaining records with excellent temporal resolution.

Europeans (mainly whalers) settled the Dennis Creek region in sparse numbers between 1631 and 1685 AD. The population increased strongly after the revolutionary war, with building of saw mills, shipyards and major road construction around 1800 AD. Wetlands were 'banked' and drained mainly after 1778 AD.

Buried cedar logs were 'mined' from the mid 18^th through the late 19^th century. Iron industry was established in 1816 AD along the uplands of Cedar Swamp Creek, North of Dennis Creek marsh.

What grows there? Vegetation in Dennis Creek marsh is dominated by Spartina alterniflora, the cord grass species which grows best in areas with twice daily flooding. The present marsh is thus largely a low to middle marsh, with relatively small, land-ward areas with higher marsh vegetation which includes Spartina patens, Typha angustifolium and Phragmites communis.

Land-wards of the salt marsh are fresh water cedar swamps along the creeks. Note the vegetational zonation in the photograph below, along the marsh edge: the highest vegetation is the fringe of fresh water Phragmites with some sedges, the 'meadow' is a small area of Spartina patens; in the background, fringing the creek, large fields of Spartina alterniflora.

Question: How to extract detailed records of relative sea level rise (RSLR) from salt marshes?

Answer: Through paleo environmental analysis of dated salt marsh sediment sequences (peat)

How to get this paleoenvironmental information?

1. derive paleo-environmental information, i.e., the distance to mean high water level for each sample (core continuously sampled: foraminifera, sediment chemistry); see figure below. Foraminiferal zonation mirrors plant zonation, but is somewhat more precise.
2. construct Marsh Paleo-Environment curve: distance to mean high water for each sample slice
3. construct age model (²¹⁰Pb, pollution record, ¹⁴C)
4. use age and paleo-environment information to construct a local mean high water rise curve
5. curves combine effects of sea level rise, tectonics, compaction, changes in tidal range
6. combine curves from various cores in each marsh, various marshes in order to try to separate local from regional - global (?) effects; e.g., look at rate of relative sea level rise during a time slice as compared to the average rate over the last 1000 years (rate-ratio).

The paleoenvironmental reconstructions can be summarized in Marsh-Paleo-Environmental (MPE) curves, which show the vertical distance of the paleo-marsh surface to a paleo sea-level plane for each sample interval.

We use foraminiferal evidence to reconstruct the MPE curves; foraminiferal zones in Dennis Creek are according to the below figure.

Foraminifera also give information on changes in overall salinity at the core site. Note that various salinity indicators agree that the lower part of the core was deposited in low salinity environments within the marsh. This fresh-water penetration ended was about 1730-1750AD.

MPE curves for three different cores in Dennis Creek marsh: the vertical axis shows depth in core for the midpoint of each sample, the horizontal axis ahows the dustance between the sample at time of deposition at the marsh surface and the level of mean high water (MHW) at that time.

Next, we must establish age-depth relations through a set of ¹⁴C and ²¹⁰Pb dated points.

We use the ²¹⁰Pb data to date the onset of metal pollution in cores. For instance, here is a figure of Pb pollution in core DCA.

Table of ¹⁴C data for Dennis Creek cores.

Sample code	depth , cm	Plant remains	d¹³C, ^o/_oo	Years BP (+ error)	Age, cal. (BC/AD)	1 s range
DCA 2	225-250	Scirpus	-27.0	230 (100)	AD 1660	1630-1690
DCA 5.2	260-275	Phragmites	-26.9	715 (120)	AD 1287	1216-1394
DCA 1	310-330	Phragmites	-27.3	810 (70)	AD 1245	1174-1285

DCL 1-2	160-174	S. patens/ Phragmites	-14.0	270 (125)	AD 1650	1481-1810
DCD 6	220-230	Phragmites	-23.0	1120 (70)	AD 960	870-1000
DCD 7	260-270	Phragmites	-17.2	1090 (70)	AD 980	890-1020
DCL 1-5	440-445	Scirpus/ S. patens	-17.7	2130 (135)	BC 167	BC 361- AD 47

DCE 3	131-150	S. patens	-18.5	600 (70)	AD 1400	1300-1420
DCE 4	200-221	S. patens/ S. alterniflora	-21.7	1400 (70)	AD 650	620-650

For each core an age model is generated based on several age index points under the assumption of a continuous sedimentary record. Thse age data are then combined with the data in the MPE curves to generate Relative Sea Level Rise curves. Note the increase in rate of relative sea level rise at about 1650 AD, and the next increase at about 1800 AD.

To assess correlations between periods with anomalous rates of RSLR in different marshes and geographic areas, the data can be expressed as fractions from their local long-term average RSLR rates. The obtained deviations from the local average rates, expressed as non dimensional rate ratios, provide evidence for periods in which the rates of RSLR were anomalous with respect to the local average rate. These rr-values can then be compared with climate indicators. such as the oxygen isotope record from the Greenland Ice core (GISP).

Note the very strong increase in rr-values, first after the end of the Little Ice Age at about 1650AD, then again after about 1800 AD.

Average rates of RSLR along the eastern US seaboard were 1-2.5 mm/a over the last 1000 - 1500 years, but tide gauge records and

sedimentological studies show that higher rates prevailed during the 20th century. These studies indicate that these higher rates of RSLR set in at the ending of the Little Ice Age at about 1650 AD, but significanbtly increased yet again at about 1800 AD.

We can also subtract the long-term average and comprae the obtained rates of sea level rise with a climate indicator.

CONCLUSIONS

Detailed paleoenvironmental analysis of salt marsh peat cores may provide information on the interplay of salt marsh accretion, geomorphological marsh development, and relative sea level rise.
Limitations are imposed by the problems of providing a precise and accurate age model, especially for the time interval where neither ¹⁴C nor ²¹⁰Pb data can be applied (about 1670 -1890 AD)
The average rate of relative sea level rise in the Dennis Creek salt marshes over the last 1200 years was ~ 2.3 mm/a.
Acceleration to ~ 5.7 mm/a started at some time between 1600 and 1700 AD, and salt water penetrated further into the marsh, changing its ecology.
At about 1800 AD, the rate of relative sea level rise further increased to ~ 8.0 mm/a.
During the Mediaeval Warming relative sea level rise was not accelerated, but the increase in the 17^th century was about coeval with the end of the Little Ice Age.