The δ13C of organic matter bound within the crystal lattice of foraminiferal calcite tests may provide a potential tracer of the isotopic composition of the surface water primary photosynthate. Using δ13C of the organic matter extracted from the crystal lattice and the calcite test, it is theoretically possible to estimate the paleo-surface water pCO2. We have tailored this technique initially for the subpolar planktonic foraminifera species Globigerina bulloides. Initial surface water pCO2 estimates from deep-sea core BOFS 5K (50°41.3′N, 21°51.9′W, water depth 3547 m) indicate that the northeast Atlantic Ocean may have been a greater sink for CO2 during the last glacial than during the Holocene. Greatly reduced benthic foraminifera abundances, especially phytodetritus feeders, in BOFS 5K during the last glacial indicates low surface productivity. This rules out a productivity-driven CO2 sink. The enhanced glacial CO2 sink must, therefore, have results from a southwards shift of the centre of deep water formation. 相似文献
We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the δ18O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19.
Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed.
The mean of the Stage 19 age estimates for the MBB is 777.9 ± 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 ± 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 ± 3.5 (N = 10) from a compilation of 40Ar/39Ar results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 ± 1.7 (N = 28).
The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface.
Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions. 相似文献
The stable isotopic δ18O and δ13C composition of foraminiferal shell calcite varies as a function of many factors including temperature and salinity. In order
to understand and interpret the variations in the isotopic composition of foraminiferal shell calcite, research has been recently
focused on the role of the “vital effects”. Our examination of the lamella structure of several recent planktonic foraminifera
indicates that the secretion of sequential lamellae results in multiple lamillae on earlier chambers and a single lamella
on the final chamber. We used a very simple procedure to separate the individual whole test of foraminifera into several chambers
and measured the isotopic composition of each growth stage chamber. The results indicate that the stable isotopic composition
(carbon and oxygen), particularly that of the last two chambers, of the foraminiferal test varies as a function of the individual
growing process.
Contribution No. 1847 from the Institute of Oceanology, Academia Sinica. 相似文献
We have examined the climatic variance in a series of deep-water oxygen-isotope records which range in length from 0.3 to 130 million years and have temporal resolutions between one thousand and 10 million years. These variations in 18O are interpreted as a generalized index of temperature change in high latitudes. Over five frequency decades the relation between log (variance density) and log (frequency) is approximately linear with a slope between –1 and –1.5. This relationship is interpreted as a background continuum of the sort postulated by Mitchell (1976) in which the spectrum is built up by layers of variance representing contributions from various processes acting within the climate system on different time scales. Our observed continuum slope is much steeper than that visualized by Mitchell. Additional variance is distributed at periods longer than about 3 million years, where it probably originates from forcing by tectonic processes; and at periods between 20,000 and 100,000 years where the Milankovitch forcing operates. Between these two regions there is a clear variance minimum which we predict will appear in the spectrum of other geological variables that are controlled by climate.A broad-band concentration of variance at periods near 30 My rises well above the background. Another concentration occurs at frequencies too low to be estimated accurately from our data. We assume this is a climatic response to the 400 My cycle of continental fragmentation and assembly. 相似文献
The distribution, structural relations and ages of ophiolites and ophiolitic melanges in the Eastern Deserts of Egypt and north-east Sudan, and their relation to arc and post-arc magmatism, are used to infer the probable positions of sutures and the directions of obduction and subduction. The ages of the successive tectonic stages in the terranes decrease northwards, by about 100 Ma in the region discussed. Obduction was north-westwards. The obducted sheets of ophiolite and ophiolitic melange, extending about 500 km north from the Onib-Sol Flamed suture, may all represent a single back-arc basin. The direction of subduction is still not clearly proved: more geochemical evidence of arc-magmatic polarity is needed. The late tectonic north-westward thrusting, the NW - SE stretching lineation, the transpressional north-south Hamisama Shear Zone and the NW-SE sinistral Najd faults are all attributed to north-westward movement (present coordinates) of terranes towards the older craton west of the Nile. The Najd faults are interpreted as intracontinental transform faults. 相似文献
Summary The Tertiary and post-Tertiary chronology of faulting in the Gregory Rift Valley is discussed with particular reference to areas where Middle Pleistocene deposits have been mapped. It is concluded that while strong faulting occurred at the end of the Middle Pleistocene period, the main faulting was earlier. At least three pre-Middle Pleistocene phases of faulting are recognised; the maximum movements probably occurred in late Pliocene or earliest Pleistocene times. 相似文献
The Sol Hamed complex, a sequence from ultramafics, through gabbros, thin sheeted dykes, to pillow lavas, is an ophiolite. It was obducted, tilted nearly to vertical, eroded and covered unconformably by the Nafirdeib Series. This is a volcaniclastic series, dated at 712 ± 58 Ma, the lower part of which includes conglomerates with ophiolitic clasts, olistostromes, one with large oolitic limestone slabs, felsitic and dacitic tuffs, turbidites and black shales, with andesites above. Some of the structures in the ophiolite are attributed to sub-oceanic deformation. Deformation after the deposition of the Nafirdeib Series produced folds and cleavage trending about NE-SW and, more locally, a cleavage trending NW in a shear zone where the ophiolite was thrust northeastwards over the Nafirdeib Series. The ophiolite and the Nafirdeib Series were intruded by the batholithic granite dated elsewhere at ca. 669 Ma. The ophiolite is thought to occur on a suture with the Nafirdeib Series representing part of an island arc sequence. 相似文献