Showing papers by "Ellen Thomas published in 2000"

Deep-sea environments on a warm earth: latest Paleocene-early Eocene

Abstract: Latest Paleocene early Eocene high-latitude surface and global deep-ocean waters were warmer than those of today by up to 15'C; planktonic foraminiferal and nannofossil assemblages suggest that primary oceanic productivity was low. Low oceanic productivity is also indicated by geochemical evidence that the supply of nutrients to the oceans may have been low. Climate modeling suggests that oceanic and atmospheric circulation may have been slu-egish at low temperature gradients, leading to low rates of upwelling of nutrients. Benthic foraminiferal data, by contrast, suggest hat the food supply to the deep sea floor in open-ocean settings was larger than that in Recent oceans. in ir,ereetnent with the speculation that a larger fraction of organic carbon u'as buried. The benthic foraminiferal evidence mi-sht be erplained b1'more elhcient food transt'er to the bottom in poorll. oxygenated. warm deep u'aters. Possiblr the pelagic microbial loop u'as more active at the higher temperatures. leading to enhanced zooplankton productivity and thus enhanced food supply. Or possibly' the benthic faunas do not indicate a high average food supply, but a more continuous and less seasonally pulsed supply than that toda,v. Environrnental interpretation of early Eocene benthic foraminiferal faunas is difficult not only because they differ substantially from Recent ones but also because the faunas had been decimated by a massive extinction during an episode of rapid warming. the Late Paleocene Thermal Maximum (LPTM), with a duration of between 25 and 200 000 ka. During the LPTM carbon isotope values of the atnospheric and oceanic carbon reservoir decreased by 2-3%u a sign of major upset in the global carbon cycle. The carbon isotope excursion could be explained b1'dissociation of methane hydrates as a consequence of warming of deep water lrasses. u'hich occurred when dominant formation of deep-intermediate waters shifted fron high to low latitudes. Methane dissociation in combination with chan-ues in ocean circulation offers a possible mechanisrn for climatic instability in the absence of polar ice caps. We lack the high-resolution, stratigraphically complete biostrati-eraphical and isotope data sets necessary to evaluate whether the early Eocene climate was unstable, but high average temperatures could reflect a warm background climate with superimposed 'hyperthermals': intervals of extremely high temperatures and

Was the late Paleocene thermal maximum a unique event

Abstract: Deep-sea benthic foraminiferal faunas were decimated by a massive extinction during an episode of rapid warming, the late Paleocene thermal maximum (LPTM), with a duration of between 50 and 200 k.y. (e.g., Thomas 1989, 1990; Kennett & Stott 1991). During the LPTM carbon isotope values of the atmospheric and oceanic carbon reservoir decreased by 2–3‰, a sign of major upset in the global carbon cycle (e.g., Kennett & Stott 1991; Koch et al. 1995; Thomas & Shackleton 1996; Beerling & Jolley 1998). A large carbon isotope excursion on such short time scales could be explained by dissociation of methane hydrates (Dickens et al. 1995, 1997; Matsumoto 1995; Kaiho et al. 1996). Possibly, the long-term warming trend of the middle Paleocene, which might have been caused by increased CO2 emissions by the North Atlantic flood basalts during the opening of the North Atlantic, was a primary cause (Eldholm & Thomas 1993). This warming was more pronounced at high latitudes and could have caused the formation of warm, thus low-density surface waters close to the poles, preventing these waters from sinking and contributing to the deep and intermediate ocean waters (Dickens et al. 1995, 1997). The low density could have been exacerbated by the increased precipitation at high latitudes during warm climates, as expected in climate models which produce deep-intermediate waters at low latitudes (Bice et al. 1997) and indicated by increased kaolinite abundance in high latitude sediments (Robert & Kennett 1994). The trigger for methane dissociation, however, is not clear and several possibilities have been explored, including surface water cooling at low latitudes as a result of volcanic eruptions (Bralower et al. 1997) and seismic disturbance of continental margins (e.g., Bains et al. 1999). Methane dissociation in combination with changes in ocean circulation offers a possible mechanism for climatic instability in the absence of polar ice caps (Thomas et al. 1999). Presently, we GFF volume 122 (2000), pp. 169–170. “Early Paleogene Warm Climates and Biosphere Dynamics”

Benthic Foraminifera and Environmental Changes in Long Island Sound

Abstract: Benthic foraminiferal faunas in Long Island Sound (LIS) in the 1940s and 1960s were of low diversity, and dominated by species of the genus Elphidium , mainly Elphidium excavatum clavatum, with common Buccella frigida and Eggerella advena . The distribution of these species was dominantly correlated with depth, but it was not clear which depth related environmental variable was most important. Differences between faunas collected in 1996 and 1997, and in the 1940s and 1960s include a strong decrease in relative abundance of Eggerella advena over all LIS, an increase in relative abundance of Ammonia beccarii in western LIS, and a decrease in species diversity. The decreased diversity suggests that environmental stress caused the faunal changes. Oxygen isotope data for E . excavatum clavatum indicate that a change in salinity is not a probable cause. Carbon isotope data suggest that the supply of organic matter to the benthos increased since the early 1960s, with a stronger increase in western LIS where algal blooms have occurred since the early 1970s, possibly as a result of nutrient input by waste water treatment plants. These blooms or the resulting episodes of anoxia/hypoxia may have played a role in the increased relative abundance of A. beccarii. There is no clear explanation for the decreased abundance of E. advena , but changes in the phytoplankton composition (thus food supply) are a possible cause. Benthic foraminiferal faunal and stable isotope data have excellent potential as indicators of physicochemical environmental changes and their effects on the biota in LIS.

Productivity control of fine particle transport to equatorial Pacific sediment

Abstract: Accumulation rates of 3He (from cosmic dust), 230Th (produced in the water column), barite (produced in the water column during decay of organic matter), and Fe and Ti (arriving with wind-borne dust) all are positively correlated in an equatorial Pacific core (TT013-PC72; 01.1°N, 139.4°W; water depth 4298 m). These accumulation rates are also positively correlated with the accumulation rates of noncarbonate material. They are not significantly correlated to the mass accumulation rate of carbonate, which makes up the bulk of the sediment. The fluctuations in accumulation rates of these various components from different sources thus must result from variations in some process within the oceans and not from variations in their original sources. Sediment focusing by oceanic bottom currents has been proposed as this process [Marcantonio et al., 1996]. We argue that the variations in the accumulation rates of all these components are dominantly linked to changes in productivity and particle scavenging (3He, 230Th, Fe, Ti) by fresh phytoplankton detritus (which delivers Ba upon its decay) in the equatorial Pacific upwelling region. We speculate that as equatorial Pacific productivity is a major component of global oceanic productivity, its variations over time might be reflected in variations in atmospheric levels of methanesulfonic acid (an atmospheric reaction product of dimethyl sulfide, which is produced by oceanic phytoplankton) and recorded in Antarctic ice cores.

Kaolinite distribution in Paleocene/Eocene boundary strata of northeastern United States and Pakistan – climatic and stratigraphic implications

Abstract: (2000). Kaolinite distribution in Paleocene/Eocene boundary strata of northeastern United States and Pakistan – climatic and stratigraphic implications. GFF: Vol. 122, No. 1, pp. 56-56.