48 research outputs found
1995, Spatial and temporal variability of late Neogene equatorial Pacific carbonate
High-resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138 provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y. The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to 10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series. Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific car bonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence in carbonate sedimentation is strongest in the precession band (19-23 k.y.) and weakest though present at lower frequencies. The second mode of variability has a consistent spatial pattern of east-west asymmetry over the past 4 m.y. only; prior to 4 Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO3 dissolution. The second mode of variability is not highly coherent with insolation. Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ 1 8 indicates that although overall correlation between carbonate and δ 1 8 is low, both modes of variability in carbonate sedimentation are coherent with δ 1 8 changes at some frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ 1 8 at the dominant insolation periods, and the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ 1 8 in both EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene
Carbon 13 in Pacific Deep and Intermediate Waters, 0-370 ka: Implications for Ocean Circulation and Pleistocene CO2
Stable isotopes in benthic foraminifera from Pacific sediments are used to assess hypotheses of systematic shifts in the depth distribution of oceanic nutrients and carbon during the ice ages. The carbon isotope differences between ∼1400 and ∼3200 m depth in the eastern Pacific are consistently greater in glacial than interglacial maxima over the last ∼370 kyr. This phenomenon of “bottom heavy” glacial nutrient distributions, which Boyle proposed as a cause of Pleistocene CO2 change, occurs primarily in the 1/100 and 1/41 kyr−1 “Milankovitch” orbital frequency bands but appears to lack a coherent 1/23 kyr−1 band related to orbital precession. Averaged over oxygen-isotope stages, glacial δ13C gradients from ∼1400 to ∼3200 m depth are 0.1‰ greater than interglacial gradients. The range of extreme shifts is somewhat larger, 0.2 to 0.5‰. In both cases, these changes in Pacific δ13C distributions are much smaller than observed in shorter records from the North Atlantic. This may be too small to be a dominant cause of atmospheric pCO2 change, unless current models underestimate the sensitivity of pCO2 to nutrient redistributions. This dampening of Pacific relative to Atlantic δ13C depth gradient favors a North Atlantic origin of the phenomenon, although local variations of Pacific intermediate water masses can not be excluded at present
Climate evolution across the Mid-Brunhes Transition
The Mid-Brunhes Transition (MBT) began ∼ 430 ka with an increase in
the amplitude of the 100 kyr climate cycles of the past 800 000 years. The
MBT has been identified in ice-core records, which indicate interglaciations
became warmer with higher atmospheric CO2 levels after the MBT, and
benthic oxygen isotope (δ18O) records, which suggest that
post-MBT interglaciations had higher sea levels and warmer temperatures than
pre-MBT interglaciations. It remains unclear, however, whether the MBT was a
globally synchronous phenomenon that included other components of the climate
system. Here, we further characterize changes in the climate system across
the MBT through statistical analyses of ice-core and δ18O
records as well as sea-surface temperature, benthic carbon isotope, and dust
accumulation records. Our results demonstrate that the MBT was a global event
with a significant increase in climate variance in most components of the
climate system assessed here. However, our results indicate that the onset of
high-amplitude variability in temperature, atmospheric CO2, and sea
level at ∼430 ka was preceded by changes in the carbon cycle, ice
sheets, and monsoon strength during Marine Isotope Stage (MIS) 14 and MIS 13.</p
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Spatial and temporal variability of late Neogene Equatorial Pacific carbonate : ODP Leg 138
High resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138
provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y.
The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to
10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of
large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate
sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing
by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series.
Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific car
bonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence
in carbonate sedimentation is strongest in the precession band (19 23 k.y.) and weakest though present at lower frequencies.
The second mode of variability has a consistent spatial pattern of east west asymmetry over the past 4 m.y. only; prior to 4
Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of
Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO₃ dissolution. The second
mode of variability is not highly coherent with insolation.
Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ¹⁸O indicates that although overall correlation
between carbonate and δ¹⁸O is low, both modes of variability in carbonate sedimentation are coherent with δ¹⁸O changes at some
frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ¹⁸O at the dominant insolation periods, and
the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ¹⁸O in both
EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene
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Refinement of a high-resolution, continuous sedimentary section for studying Equatorial Pacific Ocean paleoceanography, Leg 138
Ocean Drilling Program (ODP) Leg 138 was designed to study the late Neogene paleoceanography of the equatorial Pacific Ocean at time scales of thousands to millions of years. Crucial to this objective was the acquisition of continuous, high-resolution sedimentary records. It is well known that between successive advanced piston corer (APC) cores, portions of the sedimentary sequence often are absent, despite the fact that core recovery is often recorded as 100%. To confirm that a continuous sedimentary sequence was sampled, each of the 11 drill sites was multiple-APC-cored. At each site, continuously measured records of magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE), wet-bulk density, and digital color reflectance were used to monitor section recovery. These data were used to construct a composite depth section while at the site. This strategy often verified 100% recovery of the complete sedimentary sequence with two or three offset piston-cored holes. Here, these initial efforts have been extended to document the recovery of a complete sediment section and to investigate sources of error associated with sediment density measurements and changes in local sedimentation rates. At Sites 846 through 852, fine-scale correlation (on the order of centimeters) of the GRAPE records was accomplished using the inverse correlation techniques of Martinson et al. (1982). Having a common depth scale for all holes at each site facilitated comparison of high-resolution data from different holes. After refining the interhole correlation, GRAPE records from adjacent holes were "stacked" to produce a less noisy estimate of sediment wet-bulk density for Sites 846 through 852. The continuity of the stacked GRAPE record is confirmed with reflectance and susceptibility records. The resulting stacked GRAPE records have a temporal resolution of less than 1000 yr for the past 5 m.y. Moreover, the stacking procedure allows for development of error estimates for measurements present in more than one hole. An important advantage provided by this framework is that one can determine the range of sedimentation variability between adjacent holes at a given site. This variability is caused by local sedimentation variability and by artifacts of the coring process. We demonstrate that the depth domain changes in sedimentation variability required to correlate among adjacent holes are larger than the changes induced by time-scale tuning procedures
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Sediment depths determined by comparison of grape and logging denisty data during Leg 138
Establishing true depths of recovered sediments is critical to determining sedimentation rates for high-resolution paleoclimatic
studies. We have corrected the composite depth scale, which accounts for the entire continuous sedimentary sequence, so that sediment
depths are consistent with logging depths, or "true" depths. We accomplished this by taking advantage of dual measurements
of sediment density: in-situ wireline density logs and shipboard Gamma-Ray Attenuation Porosity Evaluator (GRAPE) density
measurements. Inverse correlation techniques (Martinson et al., 1982) were used to map the composite GRAPE density record to
the logging density record at five sites (Sites 846, 847, 849, 850, and 851). Using these mapping functions, depths on the meters
composite depth scale (mcd) were transformed to logging depths, which were then used to calculate sedimentation rates and discrete
sample depths. Tables are provided for converting mcd to logging depth (see CD-ROM, this volume). Our analysis shows that the
GRAPE and density logging data are coherent at wavelengths longer than 80 cm. This analysis indicates that logging records are
good references for establishing true depth scales in sedimentary sections and may be successful proxies for shipboard measurements
over sections having incomplete or poor core recovery, within some limitations. To estimate density variability over such sections,
we define an average gain function for the logging tool that can be used to scale logging data to account for the tool's attenuation.
Using the mcd to logging depth transformations, age models developed from cores can be applied accurately to logging dat
Derivation of Del180 from sediment core log data\u27 Implications for millennial-scale climate change in the Labrador Sea
Sediment core logs from six sediment cores in the Labrador Sea show millennial-scale climate variability during the last glacial by recording all Heinrich events and several major Dansgaard-Oeschger cycles. The same millennial-scale climate change is documented for surface water δ18O records of Neogloboquadrina pachyderma (left coiled); hence the surface water δ18O record can be derived from sediment core logging by means of multiple linear regression, providing a paleoclimate proxy record at very high temporal resolution (70 years). For the Labrador Sea, sediment core logs contain important information about deepwater current velocities and also reflect the variable input of ice-rafted debris from different sources as inferred from grain-size analysis, the relation of density and P wave velocity, and magnetic susceptibility. For the last glacial, faster deepwater currents, which correspond to highs in sediment physical properties, occurred during iceberg discharge and lasted from several centuries to a few millennia. Those enhanced currents might have contributed to increased production of intermediate waters during times of reduced production of North Atlantic Deep Water. Hudson Strait might have acted as a major supplier of detrital carbonate only during lowered sea level (greater ice extent). During coldest atmospheric temperatures over Greenland, deepwater currents increased during iceberg discharge in the Labrador Sea, then surface water freshened shortly thereafter, while the abrupt atmospheric temperature rise happened after a larger time lag of ≥ 1 kyr. The correlation implies a strong link and common forcing for atmosphere, sea surface, and deep water during the last glacial at millennial timescales but decoupling at orbital timescales
Atmospheric carbon dioxide, orbital forcing, and climate
A 340,000-year record of benthic and planktonic oxygen and carbon isotope measurements from an equatorial Pacific deep-sea core are analyzed. The data provide estimates of both global ice volume and atmospheric carbon dioxide concentration over this period. The frequencies characteristic of changes in the earth-sun orbital geometry dominate all the records. Examination of phase relationships shows that atmospheric carbon dioxide concentration leads ice volume over the orbital bandwidth, and is forced by orbital changes through a mechanism, at present not fully understood, with a short response time. Changes in atmospheric CO2 are not primarily caused by glacial-interglacial sea level changes, which had been hypothesized to affect atmospheric CO2 through the effect on ocean chemistry of changing sedimentation on the continental shelves. Instead, variations in atmospheric CO2 should be regarded as part of the forcing of ice volume changes