RISING SEA LEVELS AND

RISING SEA LEVELS AND

MARSH FORAMINIFERA ALONG THE CONNECTICUT AND HOUSATONIC RIVERS, CONNECTICUT

P. Rabinovich, E. Thomas,

E & E S, Wesleyan University

Presented at the Northeastern GSA Meeting, March 2001, Burlington VT

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

Answer: through paleo-environmental analyses of dated salt marsh sediment sequences (peat)

We use benthic foraminifera (unicelluler, eukaryotic organisms which make a shell) to derive the environment of deposition of marsh sequences. This is necessary in order to place the sediment within its specific place in the intertidal zone: low marsh or high marsh. This zonation is also possible using the remnants of vegetation (see below), but foraminifera make it possible to define the zonation more precisely. Click here for more information on marsh foraminifera, click here for more information on our methods.

METHODS:

We take a core, slice it into samples of a few cm thickness.
Study of the foraminifera gives us the distance to mean high water of each sample, which we plot in a marsh paleoenvironment (MPE) curve
We derive the age for each sample using ²¹⁰Pb and ¹⁴C data.
We derive a curve that shows how the position of Mean High Water (MHW) changed over time

Data on two Connecticut Marshes:

These marshes are both adjacent to large rivers (Connecticut, Housatonic). We derived mean high water rise curves, and compared these to similar curves from other marshes, which are not close to major rivers. The tidal range increases from east to west along the Connecticut shore of Long Island Sound, so that the Housatonic marsh (Knell's Island) formed at a larger tidal range than the Connecticut River marsh (Great Island). Note that both islands are located on the eastern side of the river mouth.

Housatonic river, Knell's Island:

We studied the foraminifera in a core, and plotted relative abundances of various species. The green colors indicate species that are most abundant in high to high middle marsh. The pink color indicates a species that is most abundant next to a major channel in the marsh. The yellow color indicates species that are common with major fresh water influx (low salinity). The dark blue indicates a species that is more abundant in low to middle marsh, the black a species that is abundant in low marsh to mudflats.

We derived the following zonation for foraminifera on Knell's island:

and we linked the zonation to the tidal record:

Which led us to the following Marsh PaleoEnvironment curve, when combined with the age information shown below:

We derived the same information for Great Island, along the Connecticut river.

Foraminiferal abundances, again with green colors indicating species that are most abundant in high to high middle marsh. The pink color indicates a species that is most abundant next to a major channel in the marsh. The yellow color indicates species that are common with major fresh water influx (low salinity). The dark blue indicates a species that is more abundant in low to midle marsh, the light blue a speciex that is abundant in low marsh to mudflats, at low salinities.

Foraminiferal zonation at Great Island (note difference in tidal range with Knell's Island):

Marsh PaleoEnvironment Curve for Great Island:

Note that the ages for the lower part of this core are not (yet) well constrained; we need to obtain additional data from that part of the core. We used the simplest choice (close to straight line) for the age model for now.

Note that in the marsh paleoenvironment curves of both marshes the development goes from low to higher marsh, with no return to lower marshes higher in the cores. This is in contrast to the situation in other marshes, which are not adjacent to major rivers, where such environmental fluctuations do occur. See for instance the MPE curve for a core from Branford Marsh:

We then use the information on the dated MPE curves to derive the curves showing the rate of mean high water rise at Knell's Island and Great Island. Note the increasing rates of relative sea level rise over time (figures in green):

The curves fit well within the family of curves derived for sets of cores along the Connecticut Shore of Long Island Sound. Cores GI (Great Island) and KI (Knell's Island) in black. Cores E, F, CY: Hammock River marshes, CT; cores GA, GD, GK: Guilford Marshes, CT. Cores PTJ1, PT10: Pattaguansett Marshes; cores BI-1, BI-5: Barn Island marshes; BFL, BFB: Branford marshes, CT.

We can derive more information than just rates of sea level, rise however. In both cores, in different rivers, we see a decrease in distance from the main channel, as indicated by a decrease in abundance of Arenoparrella mexicana, in the mid 17^th century:

We see a contemporaneous decrease in fresh water influence on the marshes, with the exception of a large spike in fresh water at arund 19oo AD, a time at which several hurricanes passed through New England, and a smaller peak at around 1950 AD. We suggest that these peaks present in two different rivers indicate periods of a large fresh water flux through the rivers.

CONCLUSIONS :

Rates of relative sea level rise (RSLR), north shore Long Island Sound: ~1 mm/yr before ~1600 AD, ~1.7 mm/yr until 1900 AD, ~3.0 mm/yr until 2000
Marshes along major rivers in Connecticut did not drown during episodes of more rapid relative sea level rise, in contrast to other marshes: sediment supply?
Marshes on east side of major rivers show less fresh water influence and increased marsh build-up since about 1600 AD: climate warmed, rate of RSLR increased
Major peak in fresh water influx ~ 1900 AD (hurricane) in marshes along the Connecticut and Housatonic River; smaller peak at about 1950 AD.
Marshes along major river might provide information on regional precipitation patterns.