Environmental changes in the western Amazônia: morphological framewor, geochemistry, palynology and radiocarbon dating data

Os sedimentos do lago Coari, de ambiente de terra firme esculpido nos depósitos do Plio-Pleistocenos, e o Acará, típico lago de várzea e ambos formados nos sedimentos quaternários da planície de inundação do médio Solimões, no oeste da Amazônia, Brasil, foram estudados para investigar as condições ambientais durante sua formação. Este estudo inclui dados da composição mineralógica, química, isótopos de Pb, palinologia, datações de radiocarbono e a configuração morfológica dos lagos obtida por imagens SRTM. As condições geológica e ambiental dos lagos variam e sugerem que suas evoluções refletem processos autogenéticos em condições de floresta úmida e chuvosa. Embora caulinita, quartz, muscovita, illita e esmectita sejam os principais minerais em ambos os lagos, a geoquímica indica fonte distinta, os sedimentos do lago Acará têm maior concentração de Al2O3, Fe2O3, FeO, CaO, K2O, MgO, Na2O, P2O5, Ba, V, Cu, Ni, Zn, Pb, Sr, Li, Y e La e têm mais Pb radiogênico que os sedimentos do lago Coari. As idades de radiocarbono sugerem que há aproximadamente 10160 anos AP o lago Coari iniciou o desenvolvimento devido a avulsão do rio Solimões, enquanto o lago Acará foi formado devido ao abandono de meandro do rio Solimões e retendo o domínio das gramíneas nas suas praias há aproximadamente 3710 anos AP.

Radiocarbon Dating, Chronologic Framework, and Changes in Accumulation Rates of Holocene Estuarine Sediments from Chesapeake Bay

Rapidly accumulating Holocene sediments in estuaries commonly are difficult to sample and date. In Chesapeake Bay, we obtained sediment cores as much as 20min length and used numerous radiocarbon ages measured by accelerator mass spectrometry methods to provide the first detailed chronologies of Holocene sediment accumulation in the bay. Carbon in these sediments is a complex mixture of materials from a variety of sources. Analyses of different components of the sediments show that total organic carbon ages are largely unreliable, because much of the carbon (including coal) has been transported to the bay from upstream sources and is older than sediments in which it was deposited. Mollusk shells (clams, oysters) and foraminifera appear to give reliable results, although reworking and burrowing are potential problems. Analyses of museum specimens collected alive before atmospheric nuclear testing suggest that the standard reservoir correction for marine samples is appropriate for middle to lower Chesapeake Bay. The biogenic carbonate radiocarbon ages are compatible with 210Pb and 137Cs data and pollen stratigraphy from the same sites. Post-settlement changes in sediment transport and accumulation is an important environmental issue in many estuaries, including the Chesapeake. Our data show that large variations in sediment mass accumulation rates occur among sites. At shallow water sites, local factors seem to control changes in accumulation rates with time. Our two relatively deep-water sites in the axial channel of the bay have different long-term average accumulation rates, but the history of sediment accumulation at these sites appears to reflect overall conditions in the bay. Mass accumulation rates at the two deep-water sites rapidly increased by about fourfold coincident with widespread land clearance for agriculture in the Chesapeake watershed.

Effects of sample mass and macrofossil type on radiocarbon dating of arctic and boreal lake sediments

Dating lake sediments by accelerator mass spectrometry (AMS) 14C analysis of terrestrial plant macrofossils overcomes one of the main problems associated with dating bulk sediment samples, i.e., the presence of old organic matter. Even so, many AMS dates from arctic and boreal sites appear to misrepresent the age of the sediment. To understand the nature of these apparent dating anomalies better, we conducted a series of 14C dating experiments using samples from Alaskan and Siberian lake-sediment cores. First, to test whether our analytical procedures introduced a sample-mass bias, we obtained 14C dates for different-sized pieces of single woody macrofossils. In these sample-mass experiments, statistically equivalent ages were found for samples as small as 0.05 mg C. Secondly, to assess whether macrofossil type influenced dating results, we conducted sample-type experiments in which 14C dates were obtained for different macrofossil types sieved from the same depth in the sediment. We dated materials from multiple levels in sediment cores from Upper Capsule Lake (North Slope, northern Alaska) and Grizzly Lake (Copper River Basin, southern Alaska) and from single depths in other records from northern Alaska. In several of the experiments there were significant discrepancies between dates for different plant tissues, and in most cases wood and charcoal were older than other macrofossil types, usually by several hundred years. This pattern suggests that 14C dates for woody macrofossils may misrepresent the age of the sediment by centuries, perhaps because of their longer terrestrial residence time and the potential in-built age of longlived plants. This study identifies why some 14C dates appear to be inconsistent with the overall age-depth trend of a lake-sediment record, and it may guide the selection of 14C samples in future studies.

Radiocarbon dating of modern peat profiles: Pre- and post-bomb C-14 variations in the construction of age-depth models

We present studies of 9 modern (up to 400-yr-old) peat sections from Slovenia, Switzerland, Austria, Italy, and Finland. Precise radiocarbon dating of modern samples is possible due to the large bomb peak of atmospheric 14C concentration in 1963 and the following rapid decline in the 14C level. All the analyzed 14C profiles appeared concordant with the shape of the bomb peak of atmospheric 14C concentration, integrated over some time interval with a length specific to the peat section. In the peat layers covered by the bomb peak, calendar ages of individual peat samples could be determined almost immediately, with an accuracy of 23 yr. In the pre-bomb sections, the calendar ages of individual dated samples are determined in the form of multi-modal probability distributions of about 300 yr wide (about AD 16501950). However, simultaneous use of the post-bomb and pre-bomb 14C dates, and lithological information, enabled the rejection of most modes of probability distributions in the pre-bomb section. In effect, precise age-depth models of the post-bomb sections have been extended back in time, into the wiggly part of the 14C calibration curve.

Radiocarbon dating, sedimentation rate, granulometry and organic carbon content of ODP Leg 182 sites

This data report presents sedimentological (grain size) and geochemical (X-ray diffraction, total organic carbon, accelerator mass spectrometry radiocarbon, and percent carbonate) information obtained from the western transect (Sites 1132, 1130, and 1134) and the eastern transect (Sites 1129, 1131, and 1127) in the Great Australian Bight during Leg 182. The purpose is to quantify changing rates of sediment accumulation and changes in sediment type from the late Pleistocene and Holocene, in order to relate these changes to the well-known sea level curve that exists for this time frame. Ultimately, these data can be used to more effectively interpret lithologic variations deeper in the Pleistocene succession, which most likely represent orbitally forced sea level events.

Palynology and radiocarbon dating on sediment profile PG1756 from Lake Billyakh, NE Siberia, Russia

In this study a radiocarbon-dated pollen record from Lake Billyakh (65°17'N, 126°47'E; 340 m a.s.l.) in the Verkhoyansk Mountains was used to reconstruct vegetation and climate change since about 15 kyr BP (1 kyr=1000 cal. yr). The pollen record and pollen-based biome reconstruction suggest that open cool steppe and grass and sedge tundra communities with Poaceae, Cyperaceae, Artemisia, Chenopodiaceae, Caryophyllaceae and Selaginella rupestris dominated the area from 15 to 13.5 kyr BP. On the other hand, the constant presence of Larix pollen in quantities comparable to today's values points to the constant presence of boreal deciduous conifer trees in the regional vegetation during the last glaciation. A major spread of shrub tundra communities, including birch (Betula sect. Nanae), alder (Duschekia fruticosa) and willow (Salix) species, is dated to 13.5-12.7 kyr BP, indicating a noticeable increase in precipitation toward the end of the last glaciation, particularly during the Allerød Interstadial. Between 12.7 and 11.4 kyr BP pollen percentages of herbaceous taxa rapidly increased, whereas shrub taxa percentages decreased, suggesting strengthening of the steppe communities associated with the relatively cold and dry Younger Dryas Stadial. However, the pollen data in hand indicate that Younger Dryas climate was less severe than the climate during the earlier interval from 15 to 13.5 kyr BP. The onset of the Holocene is marked in the pollen record by the highest values of shrub and lowest values of herbaceous taxa, suggesting a return of warmer and wetter conditions after 11.4 kyr BP. Percentages of tree taxa increase gradually and reach maximum values after 7 kyr BP, reflecting the spread of boreal cold deciduous and taiga forests in the region. An interval between 7 and 2 kyr BP is noticeable for the highest percentages of Scots spine (Pinus subgen. Diploxylon), spruce (Picea) and fir (Abies) pollen, indicating mid-Holocene spread of boreal forest communities in response to climate amelioration and degradation of the permafrost layer.