(Groton, CT, October 2002)
Published 2004, pages
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The paleo-environmental history of Long Island Sound
as traced by organic carbon, biogenic silica
and stable isotope / trace element studies in sediment cores
Varekamp1, J. C., Thomas1, E., Lugolobi, F., and Buchholtz ten Brink2, M. R.
1 Department of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, CT 06459
2Center for Coastal and Marine Geology, U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543
The bottom waters of western and central Long Island Sound (LIS) have suffered seasonal hypoxia and anoxia over the last 30 years (Parker and O'Reilly, 1991). Oxygen-depleted bottom waters are an expression of a perturbed ecosystem and deserves a thorough study regarding its potential causes (Koppelman et al., 1976; O'Shea and Brosnan, 2000). The eutrophication of the Sound over the last 150 years (Lugolobi et al, this volume; Thomas et al., this volume) is commonly seen as the cause of the hypoxia in LIS and other coastal waters, but other parameters such as water temperature may have an impact as well. The severity of hypoxia is loosely correlated with climatic conditions such as El Niño events and ambient summer temperatures (Wilson, pers. comm.).
The effect of high summer temperatures on LIS can be summarized (e.g., Welsh and Eller, 1991) as follows:
If organic matter is available for decomposition, the ambient temperature has a significant influence on the rate of oxygen consumption in the bottom water column. Long Island Sound may have suffered from hypoxia in the past, prior to the fertilization of the Sound waters by anthropogenic nutrient releases, as a result of high water temperatures.
We present data that can be used as proxies for organic productivity in the Sound: Corg and biogenic silica accumulation rates. In addition, we reconstruct the paleo-temperature and paleo-salinity history of the Sound for the last 1000 years using stable isotope and trace element data from carbonates in sediment cores. We thus provide a historic reference framework of paleo-environmental conditions in which we can place the environmental problems in LIS waters of the last 50 years (similar to the work of Cooper and Brush, 1993, for Chesapeake Bay). An extension of this work is the study of carbon isotopes on the same carbonate samples (Lugolobi et al., this volume).
We determined Corg concentrations with a carbon analyzer and biogenic silica concentrations through extractions with hot alkaline solutions. Cores were dated with radiogenic isotopes and age matching with Hg pollution profiles that have been dated extensively in the coastal marshes of Connecticut (Varekamp et al., 2000; 2003). Bulk dry densities of the sediment samples were derived from measured water contents and assumed rock densities of 2.6 gr/cm3. The stable isotope and trace element studies were carried out on the tests of the foraminiferal species Elphidium excavatum (Thomas et al., this volume; 2000), which were picked from both surface and core sediment samples. Living specimens in surface samples were detected through Rose Bengal staining. Cores were taken along transects through LIS (Buchholtz ten Brink et al., 2000; Thomas et al., this volume ).
We developed a simple mixing model for LIS waters based on mixing of Atlantic sea water and river water, with salinity as the mixing parameter. From the properties of the two end-members we derived a relationship between salinity and the oxygen isotopic composition of water. We use the Ca/Mg values in calcite as a proxy for temperature (e.g., Lea, 1999), as preliminarily calibrated on the core tops using a mean annual LIS water temperature of 12.5 oC. The Mg/Ca in calcite does not depend on salinity in the range of modern LIS salinities. The Mg/Ca paleo-temperatures were used to calculate d18Ow from d 18Occ using the fractionation factors of Kim and O'Neil (1997) and a species specific offset of 1.1 o/oo (as determined from surface sample and water column studies). The obtained d 18Ow values wee then translated into paleosalinities using the modern LIS mixing model.
We studied cores A1C1 (WLIS), D3C2 (Central LIS) and WLIS75 (Narrows; Thomas et al., this volume ) for Corg and biogenic silica (BSi) concentrations, which were recalculated into BSi and Corg accumulation rates (mg/cm2yr) for cores D3C2 and A1C1 (Figures 1, 2). The results show an increase in Corg and BSi accumulation rates since about 1800-1850, with a 5-6 times increase in the midst of the 20th century compared to pre-colonial times, and very high values in the most recent sediments. Both BSi and Corg show an exponential increase in accumulation rate over time in core A1C1. Core D3C2 shows steadily increasing Corg accumulation rates over the last 150 years, whereas BSi accumulation rates show a strong increase around the 1970's. The BSi contents of core WLIS75 (Figure 3) are overall high (~ 3.5 %) relative to pre-colonial values (~0.5 %), but show a strong drop in the 1980's-early 1990's. That drop correlates with changes in the benthic foraminiferal faunas in the core (Thomas et al., this volume).
The paleo-temperature data from Ca/Mg values (Figure 4) were determined for core A1C1, which shows a period of high temperatures about 1000 years ago, followed by a cooler interval (with two peaks between 1500 and 1900) and a period of warming over the last century. This temperature pattern shows a strong resemblance to an oxygen isotope record from the GISP2 ice core (GISP2 CD), which we translated into an approximate temperature scale for the northern hemisphere. We recognize the Mediaeval Warm Period (MWP), Little Ice Age (LIA) and Modern Global Warming (MGW) trends in both records. The two peaks in the 1500-1900 time interval in the A1C1 record are much more pronounced than in the ice core record, which may be the result of poor temperature calibration or a true regional signal as compared to the much larger-scale record in the ice core.
The paleo-salinity record of core A1C1 (Figure 5) shows a maximum during the MWP and lower values during the LIA (wet and cool). There is no secular trend apart from the high salinity period about 1000 years ago (a warm and dry MWP).
The BSi and Corg data in western and central LIS show strong eutrophication , which started around 1850 AD. The carbon loading apparently increased unabatedly over the last 100 years. Core WLIS75 shows evidence for decreasing amounts of BSi in the last 20 years of the 20th century, indicating reduced productivity of diatoms. Ecological studies suggest that a switch from diatoms to dinoflagellates may have occurred in recent times (Thomas et al., this volume; Capriulo et al., 2002).
The paleo-temperature record shows a clear pattern of paleo-climate and the paleo- salinity pattern suggests alternating 'dry and hot' and 'wet and cool' climate periods. Our data suggest that the carbon loading in western LIS exerts a strong control on the occurrence of hypoxia: the hypoxic periods started when the amount of organic carbon in the sediment cores went up dramatically (see also Lugolobi et al., this volume). Nonetheless, the rising temperature trend also correlates with the occurrence of common summer hypoxia, and temperature may have played a subsidiary role in its development.
Funding for this research was provided by the CT SeaGrant College Program, the EPA Long Island Sound Office, the USGS and the CTDEP-administered Lobster Research Fund.
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Lea, D.W., 1999, Trace elements in foraminiferal calcite. In: B. K. Gupta (ed): Modern Foraminifera. 259-277, Kluwer Academic Publishers.
Lugolobi, F., Varekamp, J. C., Thomas, E., and Buchholtz ten Brink,M.R., this volume, The use of stable carbon isotopes in foraminiferal calcite to trace changes in biological oxygen demand in Long Island Sound.
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Varekamp, J. C., Kreulen, B., Buchholtz ten Brink, M. R., and Mecray, E. L., 2003. Mercury contamination chronologies from Connecticut wetlands and Long Island Sound sediments. Environmental Geology 43, 268-282, DOI: 10.1007/s00254-002-0624-x
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Figure 1. Organic Carbon and Biogenic Silica accumulation rates in core A1C1. The rates start to increase around 1800-1850 AD and increase exponentially over the last 50 years.
Figure 2. Organic Carbon and Biogenic Silica accumulation rates in core D3C2. The carbon accumulation rate increases steadily since 1850 AD, whereas BSi increases strongly in the 1980's.
Figure 3. Biogenic Silica concentrations in core WLIS75. The concentrations are the highest found among LIS samples, but drop in the 1980's.
Figure 4. Paleo-temperature record of core A1C1. The Mediaeval Warm Period (MWP), Little Ice Age (LIA) and Modern Global Warming (WGW) intervals are clearly recognizable. For comparison, the GISP2 record is plotted with a non-calibrated temperature scale.
Figure 5. Paleo-salinity record of core A1C1, with salinities calculated based on the Ca/Mg water temperatures (TW). The data indicate a high salinity at the MWP (hot and dry) and a lower salinity during the cooler LIA period.