926 resultados para Event-based timing
Resumo:
Late Maestrichtian to late Eocene bathyal benthic foraminiferal faunas at Sites 752,753, and 754 on Broken Ridge in the eastern Indian Ocean were analyzed as to their stratigraphic distribution of species to clarify the relation between faunal turnovers and paleoceanographic changes. Based on Q-mode factor analysis, eight varimax assemblages were distinguished: the Stensioina beccariiformis assemblage in the upper Maestrichtian to upper Paleocene; the Cibicidoides hyphalus assemblage in the upper Maestrichtian; the Cibicidoides cf. pseudoperlucidus assemblage in the upper Paleocene; the Anomalinoides capitatusldanicus assemblage in the uppermost Paleocene to lower Eocene; the Cibicidoides subspiratus assemblage in the lower Eocene; the Nuttallides truempyi assemblage in the lower and middle Eocene; the Osangularia sp. 1 - Hanzawaia ammophila assemblage in the upper Eocene; and the Lenticulina spp. assemblage in the uppermost Eocene, Oligocene, and lower Miocene. The presence of the Osangularia sp. 1 - Hanzawaia ammophila assemblage is related to the shallowing episode on Broken Ridge (upper bathyal), as a result of the rifting event that occurred in the middle Eocene. The most distinct faunal change (the disappearance of about 37% of the species) occurred between the S. beccariiformis assemblage and the A. capitatusldanicus assemblage, at the end of the upper Paleocene. A. capitatusldanicus, Lenticulina spp., and varied forms of Cibicidoides replaced the Velasco-type fauna at this time. The timing of this event is well correlated with the known age at South Atlantic sites (Thomas, 1990 doi:10.2973/odp.proc.sr.113.123.1990; Kennett and Stott, 1990 doi:10.2973/odp.proc.sr.113.188.1990; Katz and Miller, 1990 doi:10.2973/odp.proc.sr.114.147.1991). The primary cause of the extinction of the Stensioina beccariiformis assemblage is elusive, but may have resulted from the cessation of deep-water formation in the Antarctic (Katz and Miller, 1990), and subsequent arrival of warm saline deep water (Thomas, 1990; Kennett and Stott, 1990). Another possibility may be a weakened influence of high-salinity water formed at the low latitudes such as the Tethys Sea. The extinction event corresponds to the change from higher delta13C values in benthic foraminifers to lower ones. An interpretation of delta13C values is that the eastern Indian deep water, characterized by young and nutrient-depleted water, became old water which was devoid of a supply of new water during the latest Paleocene to early Eocene. Prior to this benthic event, signals of related faunal change were detected in the following short periods: early and late Paleocene, near the boundary of nannofossil Zone CP4, and Zone CP5 of the late Paleocene at Site 752. Among common taxa in the upper Maestrichtian, only seven species disappeared or became extinct at the Cretaceous/ Tertiary boundary at Site 752. The benthic foraminiferal population did not change for up to 2 m above the boundary, in contrast to the rapid decrease of the plankt onic foraminiferal population at the boundary. A decrease in the number of benthic foraminifers occurs after that level, corresponding to an interval of decreased numbers of planktonic foraminifers and higher abundance of volcanic ash. Reduced species diversity (H') suggests a secondary effect attributable to the dissolution of foraminiferal tests. The different responses of planktonic and benthic foraminifers to the event just above the boundary suggest that the Cretaceous/Tertiary event was a surface event as also suggested by Thomas (1990). In addition, a positive shift of delta13C in benthic foraminifers after the event indicates nutrient-depleted bottom water at Site 752.
Resumo:
We use the oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber (white) from Ocean Drilling Program Site 1058 in the subtropical northwestern Atlantic to construct a high-resolution (~800 year) climate record spanning the mid-Pleistocene climate transition (~410 ka to 1350 ka). We investigate whether or not millennial-scale instabilities in the proxy record are associated with the extent of continental glaciation. G. ruber d18O values display high-frequency fluctuations throughout the record, but the amplitude about mean glacial and interglacial d18O values increases at marine isotope stage (MIS) 22 (880 ka) and is highest during MIS 12. These observations support that millennial-scale climate instabilities are associated with ice sheet size. Time series analysis illustrates that these variations have significant concentration of spectral power centered on periods of ~10-12 ka and ~5 ka. The timing of these fluctuations agrees well, or coincides with, the periodicities of the second and fourth harmonics, respectively, of precessional forcing at the equator. An insolation-based origin of the millennial-scale instabilities would be independent of ice volume and explains the presence of these fluctuations before the mid-Pleistocene climate transition as well as during interglacial intervals (e.g., MIS 37 and 17). Because the amplitude of the millennial-scale variations increases during the mid-Pleistocene transition, feedback mechanisms associated with the growth of large, 100-ka-paced, polar ice sheets may be important amplifiers of regional surface water hydrographic changes.
Resumo:
Timing is crucial to understanding the causes and consequences of events in Earth history. The calibration of geological time relies heavily on the accuracy of radioisotopic and astronomical dating. Uncertainties in the computations of Earth's orbital parameters and in radioisotopic dating have hampered the construction of a reliable astronomically calibrated time scale beyond 40 Ma. Attempts to construct a robust astronomically tuned time scale for the early Paleogene by integrating radioisotopic and astronomical dating are only partially consistent. Here, using the new La2010 and La2011 orbital solutions, we present the first accurate astronomically calibrated time scale for the early Paleogene (47-65 Ma) uniquely based on astronomical tuning and thus independent of the radioisotopic determination of the Fish Canyon standard. Comparison with geological data confirms the stability of the new La2011 solution back to ~54 Ma. Subsequent anchoring of floating chronologies to the La2011 solution using the very long eccentricity nodes provides an absolute age of 55.530 {plus minus} 0.05 Ma for the onset of the Paleocene/Eocene Thermal Maximum (PETM), 54.850 {plus minus} 0.05 Ma for the early Eocene ash -17, and 65.250 {plus minus} 0.06 Ma for the K/Pg boundary. The new astrochronology presented here indicates that the intercalibration and synchronization of U/Pb and 40Ar/39Ar radiometric geochronology is much more challenging than previously thought.
Resumo:
The SES_GR2_Copepod Ingestion on ciliates and phytoplankton dataset is based on samples taken during August-September 2008 in Ionian Sea, Libyan Sea, Southern Aegean Sea and Northern Aegean Sea. Ingestion rates were estimated from experiments performed at all the third priority stations during the cruise according to DoW of Sesame project. Copepods for the experiments were obtained with slow non-quantitative tows from the upper 100 m layer of the water column using 200 µm mesh size nets fitted with a large non-filtering cod end. For the grazing experiments we used the following copepod species: Clausocalanus furcatus, Oithona spp. Temora stylifera and Acartia spp according to the relevant reference (Bamstedt et al. 2000). Copepod clearance rates on ciliates were calculated according to Frost equations (Frost 1972). Ingestion rates were calculated by multiplying clearance rates by the initial standing stocks (Bamstedt et al. 2000).
Resumo:
A late Quaternary pollen record from northern Sakhalin Island (51.34°N, 142.14°E, 15 m a.s.l.) spanning the last 43.7 ka was used to reconstruct regional climate dynamics and vegetation distribution by using the modern analogue technique (MAT). The long-term trends of the reconstructed mean annual temperature (TANN) and precipitation (PANN), and total tree cover are generally in line with key palaeoclimate records from the North Atlantic region and the Asian monsoon domain. TANN largely follows the fluctuations in solar summer insolation at 55°N. During Marine Isotope Stage (MIS) 3, TANN and PANN were on average 0.2 °C and 700 mm, respectively, thus very similar to late Holocene/modern conditions. Full glacial climate deterioration (TANN = -3.3 °C, PANN = 550 mm) was relatively weak as suggested by the MAT-inferred average climate parameters and tree cover densities. However, error ranges of the climate reconstructions during this interval are relatively large and the last glacial environments in northern Sakhalin could be much colder and drier than suggested by the weighted average values. An anti-phase relationship between mean temperature of the coldest (MTCO) and warmest (MTWA) month is documented during the last glacial period, i.e. MIS 2 and 3, suggesting more continental climate due to sea levels that were lower than present. Warmest and wettest climate conditions have prevailed since the end of the last glaciation with an optimum (TANN = 1.5 °C, PANN = 800 mm) in the middle Holocene interval (ca 8.7-5.2 cal. ka BP). This lags behind the solar insolation peak during the early Holocene. We propose that this is due to continuous Holocene sea level transgression and regional influence of the Tsushima Warm Current, which reached maximum intensity during the middle Holocene. Several short-term climate oscillations are suggested by our reconstruction results and correspond to Northern Hemisphere Heinrich and Dansgaard-Oeschger events, the Bølling-Allerød and the Younger Dryas. The most prominent fluctuation is registered during Heinrich 4 event, which is marked by noticeably colder and drier conditions and the spread of herbaceous taxa.
Resumo:
Greenland ice core records indicate that the last deglaciation (~7-21 ka) was punctuated by numerous abrupt climate reversals involving temperature changes of up to 5°C-10°C within decades. However, the cause behind many of these events is uncertain. A likely candidate may have been the input of deglacial meltwater, from the Laurentide ice sheet (LIS), to the high-latitude North Atlantic, which disrupted ocean circulation and triggered cooling. Yet the direct evidence of meltwater input for many of these events has so far remained undetected. In this study, we use the geochemistry (paired Mg/Ca-d18O) of planktonic foraminifera from a sediment core south of Iceland to reconstruct the input of freshwater to the northern North Atlantic during abrupt deglacial climate change. Our record can be placed on the same timescale as ice cores and therefore provides a direct comparison between the timing of freshwater input and climate variability. Meltwater events coincide with the onset of numerous cold intervals, including the Older Dryas (14.0 ka), two events during the Allerød (at ~13.1 and 13.6 ka), the Younger Dryas (12.9 ka), and the 8.2 ka event, supporting a causal link between these abrupt climate changes and meltwater input. During the Bølling-Allerød warm interval, we find that periods of warming are associated with an increased meltwater flux to the northern North Atlantic, which in turn induces abrupt cooling, a cessation in meltwater input, and eventual climate recovery. This implies that feedback between climate and meltwater input produced a highly variable climate. A comparison to published data sets suggests that this feedback likely included fluctuations in the southern margin of the LIS causing rerouting of LIS meltwater between southern and eastern drainage outlets, as proposed by Clark et al. (2001, doi:10.1126/science.1062517).
