C. Hillaire-Marcel

Recent sedimentary history of anthropogenic impacts on the Culiacan River Estuary, northwestern Mexico: geochemical evidence from organic matter and nutrients.

210Pb geochronology and sediment profiles of carbon, phosphorus and nitrogen were used to study time dependent changes in nutrients fluxes to Culiacan River Estuary. Results indicate that the release of urban sewage and agriculture wastes transported through Culiacan River has produced historically increased carbon, phosphorus and nitrogen fluxes to the study area. C:N:P elemental ratios showed that increments in the nutrients input begins simultaneously for C, N and P in 1948 with the clearing of the catchment for agriculture; although excess of nutrients input increased most importantly around the 1970s to roughly follow the rapidly growing population of Culiacan City. C/N ratios, delta13C and delta15N suggested that nutrient enrichment is mostly influenced by sewage delivered through Culiacan River.

Quantitative assessment of carbonate dissolution in marine sediments from foraminifer linings vs. shell ratios: Davis Strait, northwest North Atlantic

A new method to evaluate quantitatively the degree of calcium carbonate preservation and dissolution in high-latitude marine sediments is proposed on the basis of relative abundance of CaCO 3 shells and organic linings of benthic foraminifers. This method was applied to a late Quaternary sequence from Davis Strait in the northwest North Atlantic and shows that CaCO 3 dissolution in sediments has increased since the last glacial maximum (ca. 18,000 B.P.) and peaked when subpolar interglacial conditions were established in surface waters. The dissolution in Davis Strait sediments appears to be closely related to organic biogenic production in surface waters, with a regional pattern of bottom-water formation and circulation.

Historical trends of metal pollution recorded in the sediments of the Culiacan River Estuary, Northwestern Mexico

The accumulation of selected trace metals (Cd, Co, Cu, Ni, Pb and Zn) was studied in a sediment core collected in a shallow sandy area at the upper estuary of the Culiacan River. The chronology was developed by using the accumulation rates determined previously from 210 Pb analyses of the same core. Trace metal concentration began to increase over background levels at depths corresponding to the late 40s and reached a maximum at the present time, excepting for Cd and Pb. The observed profile trends were related to population growth, and significant relationships between organic C and Cd, Cu and Zn indicated that such metals are mainly delivered to the estuarine sediments from a common source, identified as sewage wastes. Excess metal inventories and metal concentration factors indicated a slight pollution by all the trace metals examined, although levels of enrichment for Ni and Pb (Concentration Factors 41.5) were considerably less than those found for Co, Cd, Zn and Cu (CF’s � 3). However, the concentrations of Cu, Pb and Ni were considered as potentially toxic and therefore, these metals must be considered pollutants of concern and further investigation (e.g. biological and chemical testing) is strongly recommended. # 2002 Elsevier Science Ltd. All rights reserved.

Palynostratigraphy and chronostratigraphy of Baffin Bay deep sea cores: Climatostratigraphic implications

Abstract Palynostratigraphic considerations and 14C dates (AMS; accelerator mass spectrometry) on foraminifera samples from piston cores suggest sedimentation rates as high as ca. 8–11 cm/ka in deep Baffin Bay during the Late Pleistocene. Throughout this interval Baffin Bay experiences rigorous conditions and probably the extension of a dense sea-ice cover unfavourable to phytoplanktonic productivity: the dinocyst microflora indicates only brief episodes of higher productivity related to Arctic conditions and to moderately low salinites (ca. 30–31ℵ). The abundance of reworked pre-Quaternary palynomorphs in the deposits is interpreted as a response to glacial and glacio-marine activity in the northernmost areas surrounding Baffin Bay. The dinocyst stratigraphy spanning approximately over the last ice age (ca. 100 ka) reveals that Baffin Bay constituted a confined basin, isolated from the North Atlantic Ocean, prior to the present “Interglacial”.

Palynostratigraphy and Th/U ages of upper Pleistocene interglacial and interstadial deposits on Cape Breton Island, eastern Canada

On Cape Breton Island (Nova Scotia), near the margin of the Wisconsinan ice sheet, karst depressions in Mississippian gypsum-bearing rocks contain interglacial and interstadial organic-rich deposits. Three palynostratigraphic units have been observed and tentatively dated by Th/U measurements on embedded fossil wood. The first, dated at ca. 125 ka, may be assigned to the oceanic /sup 18/O substage 5e; thermophilous forests (Quercus, Ostrya, pinus strobus) developed in response to a climate warmer than the present. The second unit, rich in Abies balsamea pollen and dated at ca. 87 ka, may relate to the /sup 18/O substage 5a; it reflects a cool and wet climate not unlike that of today. The third unit probably spans part of the mid-Wisconsinan (/sup 18/O stage 3); it shows alternating boreal forest-tundra forest assemblages indicative of climatic oscillations during a generally cold interval.

Late holocene hydrological history of Lake Tanganyika, East Africa, from isotopic data on fossil stromatolites

Abstract Lake Tanganyika, situated in the Western Branch of the East African Rift System, shows a slightly positive water budget and generates a small outflow (through the Lukuga River), dependent mainly on the Ruzizi River inflow from Lake Kivu in the northern part of the basin. Conditions more arid than those presently observed, closed the Kivu Basin from 3800 to 1300 yr B.P. Previous studies esimated that the closure of the corresponding Tanganyika Basin lowered the lake level by 75–150 m. In this paper we present the results of isotopic and morphologic analyses of fossil stromatolites. The stromatolites formed as thick incrustations on hard substrates and are found at depths between 6 and 60 m below the modern lake level. Their isotopic composition (18O, 13C, 14C) very precisely document Late Holocene water levels and hydrologic fluctuations in the Lake Tanganyika Basin, with reference to the actual situation. A major low stand, at 10 m below present level, is identified from 3400 to 1900 yr B.P., with a rapid recession and gradual rise to modern levels. Maximum δ18O values (vs PDB) in the stromatolites rose from ca. 3.5% (at 3400 yr B.P.) to 6% (at 2500 yr B.P.), then decreased progressively to 1.6% (at 1300 yr B.P.). These values are interpreted as a maximum aridity episode culminating at ca. 2500 yr B.P., at which time a negative precipitation/evaporation (P:E) ratio resulted in a much higher residence time of paleolake waters and thus a noticeable enrichment in 18O. The observed decoupling of the isotopic ratio (18O/16O) and water volume responses is thought to be linked to the inertia of the large rift aquifers which controlled the paleolake level.

New record shows pronounced changes in Arctic Ocean circulation and climate

Does the Arctic Ocean surface circulation north of Alaska oscillate to and fro like a slow washing machine on millennial timescales? New evidence from the sediment record over the last 10,000 years suggests that it does and that in the recent past, the western Arctic Ocean was much warmer than it is today. Similar Holocene climatic fluctuations are seen in many records worldwide, yet their origin remains enigmatic. Modeling and observational studies suggest that the Arctic may play an important role in these climate fluctuations through changes in surface albedo, modifications of oceanic thermohaline circulation, and changes in biogeochemical cycling of nutrients and radiatively important gases [PARCS, 1999].

The loading history of trace metals and nutrients in Altata-Ensenada del Pabellón, lagoon complex, northwestern Mexico

This paper summarizes the geochemical investigations about the origin and loading history of some trace metals (Ag, Cu and Zn) and nutrients (N and P) in the coastal lagoon complex of Altata-Ensenada del Pabellón, Mexico, by using the radioactive chronometers 210Pb and 228Th and the stable isotopes of C and N. The examination of sediment cores collected at different locations in the lagoon system identified a slight enrichment in metals and nutrients in some points, which was mainly associated to organic matter accumulation. Stable C and N isotope ratios revealed wastewater inputs to the lagoon system and the 210Pb geochronology showed that anthropogenic impact started 50 years ago, with the beginning of the agriculture development and the associated urban growth of the surrounding area. Several atypical 210Pb and 228Th/232Th profiles demonstrated that biological and physical disturbances are common phenomena in these environments, that frequently mask the pollution records; and therefore, considering that the contaminated sediments at some locations in the lagoon system are frequently resuspended and re-oxygenated, the pollutants will continue to be easily remobilized in the food chain.

The effect of speleothem surface slope on the remanent magnetic inclination

Speleothems are of interest for high-resolution reconstruction of the Earths magnetic field. However, little is known about the influence of speleothem morphologies on their Natural Remanent Magnetization (NRM) record. Here we report on a high-resolution paleomagnetic study of a dome-shaped speleothem of Middle Holocene age from southern Portugal, with special attention to the anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). To assess the potential influence of the slope of the speleothem surface on the recorded remanent magnetization, we compare magnetic directions and AMS and AARM fabrics from sub-horizontal to gradually sub-vertical calcite growth layers collected in a transversal cross-section of the speleothem. A linear correlation is observed between magnetic inclinations, calcite laminae slope and AARM k1 inclination. The AMS fabric is mostly controlled by calcite crystals, with direction of the minimum axes (k3) perpendicular to laminae growth. Magnetic inclinations recorded in inclined and vertical calcite growth layers are underestimated when compared to a global paleosecular variation (PSV) model. After extrapolating magnetic inclinations to the horizontal, the corrected data better fit the PSV model, but are still lower than the predicted magnetic inclinations, suggesting that inclination shallowing affects the entire speleothem. We suggest that speleothem morphology exerts a critical role on the magnetic inclination recording, which is controlled by the Earths magnetic field but also influenced by particle rolling along the sloping surfaces. These observations open new avenues for reconstructing high-resolution paleomagnetic secular variations records from speleothems and provide new insights into their NRM acquisition mechanisms.

A New Chronology of Late Quaternary Sequences From the Central Arctic Ocean Based on “Extinction Ages” of Their Excesses in 231 Pa and 230 Th: LATE QUATERNARY ARCTIC OCEAN CHRONOLOGY

Merging the late Quaternary Arctic paleoceanography into the Earth’s global climate history remains challenging due to the lack of robust marine chronostratigraphies. Over ridges notably, low and variable sedimentation rates, scarce biogenic remains ensuing from low productivity and/or poor preservation, and oxygen isotope and paleomagnetic records differing from global stacks represent major impediments. However, as illustrate here based on consistent records from Mendeleev-Alpha and Lomonosov Ridges, disequilibria between U-series isotopes can provide benchmark ages. In such settings, fluxes of the particle-reactive U-daughter isotopes Th and Pa from the water column, are not unequivocally linked to sedimentation rates, but rather to sea-ice rafting and brine production histories, thus to the development of sea-ice factories over shelves during intervals of high relative sea level. The excesses in Th and Pa over fractions supported by their parent U-isotopes, collapse down sedimentary sequences, due to radioactive decay, and provide radiometric benchmark ages of approximately 300 and 140 ka, respectively. These ‘‘extinction ages’’ point to mean sedimentation rates of 4.3 and 1.7 mm/ka, respectively, over the Lomonosov and Mendeleev Ridges, which are significantly lower than assumed in most recent studies, thus highlighting the need for revisiting current interpretations of Arctic lithostratigraphies in relation to the global-scale late Quaternary climatostratigraphy. Plain Language Summary The Arctic Ocean represents a major component of the Earth climate system notably with regard to the Arctic amplification and freshwater fluxes toward the global ocean. Understanding its role versus the global climate history of the recent glacial/interglacial cycles remains challenging due to the lack of robust chronology of marine sedimentary archives. In the present study, we demonstrate that the decay of Uranium series isotopes in sediments from major Arctic ridges provide benchmark ages for the last 300,000 years and support the concept of a ‘‘sediment-starved’’ environment in the central Arctic Ocean.