Gamma Ray Attenuation Porosity Evaluator Research Articles

A revised mid-Pliocene composite section centered on the M2 glacial event for ODP Site 846

Abstract. The composite section from ODP Site 846 has provided key data sets for Pliocene stable isotope and paleoclimatic time series. We document here apparent outliers in previously published data sets for stable isotopes and alkenone-derived sea surface temperature (SST) estimates in the Pliocene interval containing the M2 glaciation (ca. 3.290–3.3 Ma) by tying high-resolution core measurements to a continuous downhole conductivity log. We generate a revised sequence of new stable isotopic and alkenone measurements across the M2 event that correlate well to the revised splices of color reflectance and gamma ray attenuation porosity evaluator data from Site 846, and to a new composite section produced at equatorial Pacific ODP Site 850. A new composite splice for Site 846 is proposed, along with composite isotope and alkenone time series that should be integrated into revised Pliocene paleoclimatic stacks.

The relationship between wet bulk density and carbonate content in sediments from the Eastern Equatorial Pacific

Abstract Sediment cores collected from the Eastern Equatorial Pacific Ocean display a clear positive second-order relationship between wet bulk density (WBD) and carbonate content. This has long interested the paleoceanography community because detailed Gamma Ray Attenuation Porosity Evaluator (GRAPE) measurements, which approximate WBD, might be used to determine records of carbonate content at very high temporal resolution. Although general causes for the relationship are known, they have not been presented and discussed systematically on the basis of first principles. In this study, we measure the mass and carbonate content of 50 sediment samples with known WBD from Site U1338, before and after rinsing with de-ionized water; we also determine the mass related proportion of coarse (> 63 μm) material. Samples exhibit clear relationships between WBD, carbonate content, mass loss upon rinsing, and grain size. We develop a series of mathematical expressions to describe these relationships, and solve them numerically. As noted by previous workers, the second-order relationship between WBD and carbonate content results from the mixing of biogenic carbonate and biogenic silica, which have different grain densities and different porosities. However, at high carbonate content, a wide range in WBD occurs because samples with greater amounts of coarse carbonate have higher porosity. Moreover compaction impacts carbonate particles more than biogenic silica particles. As such, a single two-component equation cannot be used to determine carbonate content accurately across depth intervals where both the porosity and type of carbonate vary. Instead, the WBD-carbonate relationship is described by an infinite series of curves, each which represents mixing of multiple sediment components with different densities and porosities. Dissolved ions also precipitate from pore space during sample drying, which adds mass to the sediment. Without rinsing samples, simple empirical relationships between WBD and carbonate content are further skewed by salt dilution.

Destructive and non-destructive density determination: method comparison and evaluation from the Laguna Potrok Aike sedimentary record

Density measurements play a central role in the characterization of sediment profiles. When working with long records (>100 m), such as those routinely obtained within the frame of the International Continental Scientific Drilling Program, several methods can be used, all of them varying in resolution, time-cost efficiency and source of errors within the measurements. This paper compares two relatively new non-destructive densitometric methods, CT-Scanning and the coherent/incoherent ratio from an Itrax XRF core Scanner, to data acquired from a Multi-sensor core logger Gamma Ray Attenuation Porosity Evaluator (MSCL Grape) and discrete measurements of dry bulk density, wet bulk density and water content. Quality assessment of density measurements is performed at low and high resolution along the Laguna Potrok Aike (LPA) composite sequence. Giving its resolution (0.4 mm in our study), its high signal to noise ratio, we conclude that CT-Scan provides a precise, fast and cost-efficient way to determine density variation of long sedimentary record. Although more noisy that the CT-Scan measurements, coherent/incoherent ratio from the XRF core scanner also provides a high-resolution, reliable continuous measure of density variability of the sediment profile. The MSCL Grape density measurements provide actual density data and have the significant advantage to be completely non-destructive since the acquisition is performed on full cores prior to opening. However, the quality MSCL Grape density measurements can potentially be reduced by the presence of voids within the sediment core tubes and the dry and bulk density measurements suffers from sampling challenges and are time-consuming.

Late Neogene-Quaternary radiolarian biostratigraphy: a brief review

Abstract. Since the 1950s, it has become apparent that Radiolaria have significant biostratigraphical potential throughout Phanerozoic time, including the Late Neogene and Quaternary. Radiolarian biozonation schemes for this period have been developed, including a Standard Tropical Zonation, which illustrates the pan-oceanic application of radiolarian biostratigraphy to Pliocene–Quaternary sediments. The biostratigraphical resolution obtainable using Radiolaria is equivalent to other microfossil groups, such as planktonic foraminifera. The recognition of abundance events of Cycladophora davisiana, and of some other species, are an alternative radiolarian dating technique for the Pliocene–Quaternary, akin to dating sediment using oxygen stable isotope (δ18O) records and with similar resolution. A number of studies have used astronomical timescales, derived from orbitally tuning δ18O and gamma ray attenuation porosity evaluator (GRAPE) records, to provide ages for radiolarian biodatums. This approach should be adopted as a more accurate alternative to palaeomagnetic chronologies with their inherent flaws. This commentary concludes that Radiolaria are important microfossils and, as a group, continue to offer significant potential as a biostratigraphical tool in future studies of the marine Pliocene–Quaternary.

An astronomical tuning strategy for Pliocene sections: implications for global-scale correlation and phase relationships

Astronomical tuning plays a central role in developing the chronostratigraphy of marine sediment sections. For Pleistocene sections, tuning is typically accomplished by matching the 41 and 23ka components of oxygen isotopes (δ18O−41, δ18O−23) to orbital obliquity and precession with constant phase lags of 69 degrees and 78 degrees, respectively (depleted (δ18O−41 lags maximum obliquity by 7.8ka, while depleted δ18O−23 lags minimum precession by 5ka). This approach places all records on the same time–scale relative to one another and relative to insolation forcing because Pleistocene δ18O is globally correlative. Thus, lead and lag relationships among climate variables measured at globally distributed sites can be assessed in an effort to understand the underlying physics of climate change. For Pleistocene sections, variables such as dust content, magnetic susceptibility, colour reflectance, and gamma–ray attenuation porosity evaluator (GRAPE) are typically tuned directly to precession or to a precession–dominated insolation target. Unlike Pleistocene δ18O, these variables are not globally correlative and few have unambiguous models linking them to insolation forcing. Consequently, the capacity to compare Pliocene lead and lag relationships among climate variables from different sites is considerably diminished, as is the capacity to evaluate the climate response to orbital forcing. Here a Pliocene tuning strategy is presented which builds on the precession–based approach by incorporating a final tuning step involving (δ18O−41. Results from application to Site 659 and the Mediterranean Rossello Composite Section suggest qualified success. Application to other Pliocene sections is required to verify the utility of this astronomical tuning approach.

A record of glacial/interglacial alternations in Pleistocene sediments off New Jersey expressed by clay mineral, grain-size and magnetic susceptibility data

Clay mineral, grain-size and magnetic susceptibility measurements have been performed on a high resolution sampling of Pleistocene clayey silts and silty clays drilled at ODP Site 902 (Hole D) located on the upper slope off New Jersey (NW Atlantic). The combination of data from different techniques, and their comparison with δ 18 O and Gamma-Ray Attenuation Porosity Evaluator (GRAPE) data, allow us to clearly distinguish between sediments deposited during glacial and interglacial cycles. Glacial sediments are poorly-sorted, rich in either clay or sand, and characterized by the abundance of chlorite and high magnetic susceptibility values. In contrast, interglacial sediments are well-sorted, silt-dominated and poor in chlorite. The Hudson River and its tributaries draining the Appalachian Highlands appear to constitute the perennial sources of the detrital input including quartz, feldspars, amphibole and a clay fraction dominantly composed of illite, random mixed-layers and kaolinite. During glacial stages, the terrigenous sedimentation was enhanced by additional input of chlorite from glacial erosion in Northeastern America. The variations in chlorite amounts supplied to the ocean were probably controlled by the ice-sheet advances and retreats. These variations are quantified by the kaolinite/chlorite ratio (K/C) which constitutes a reliable marker of glacial and interglacial influences on the terrigenous sedimentation.

Physical properties of Reykjanes Ridge sediments and their linkage to high‐resolution Greenland Ice Sheet Project 2 ice core data

Five gravity cores taken from the Reykjanes Ridge have been used to establish a link between sediment physical properties and atmospheric records documented by δ18O variations in Greenland ice cores over the last 45,000 calendar years. Marine Gamma Ray Attenuation Porosity Evaluator density and magnetic susceptibility variations could be linked with the ice core Dansgaard‐Oeschger and Bond cycles. This is supported by ice‐rafted detritus (IRD), grain size, the quartz/feldspar ratio, and carbonate, isotopic, and foraminiferal records. The covariation of the sediment physical properties and δ18O in Greenland ice indicates a coupling of atmospheric temperature and paleocirculation variations. Gradual reduced bottom currents (Iceland‐Scotland Overflow Water) and enhanced iceberg discharges have been reconstructed for cold atmospheric periods relative to interstadial times. In the study area the magnetic susceptibility signal is not related to the ice‐rafted detritus input but most probably reflects the variations of the Iceland‐Scotland Overflow Water intensity transporting titanomagnetite into the Reykjanes Ridge region.

Linking physical property records of Quaternary sediments to Heinrich events

Ice rafted debris in high latitude ocean sediments represent a complex record of the changing paleoenvironment of the oceans and, in particular, of the growth and decay of ice sheets along the margins of high latitudes. Physical properties measured on sediment cores taken from the Rockall Plateau were examined to determine the distribution of ice rafted debris layers and Heinrich events in the northeastern North Atlantic. These sediment core records may provide one of the keys to reconstruct the iceberg flow between the northeastern Atlantic and the Norwegian Sea. Magnetic susceptibility (MS) and Gamma Ray Attenuation Porosity Evaluator (GRAPE) density changes of these cores revealed that since about 65 ka, dropstone layers are recorded in both MS and GRAPE data of Rockall Plateau sediments. Rockall Plateau sediments show peaks in physical properties that correlate with Heinrich events (H 1, H 2, H 4, H 5, H 6). Heinrich layer 3 was not observed. The stratigraphy and physical properties represent the Heinrich layers: H 1 = 14–15 ka (MS = 52 μcgs, ϱ = 1.64 g/cm 3), H 2 = 23 ka (MS = 64 μcgs, ϱ = 1.8 g/cm 3), H 4 = 41 ka (MS = 53 μcgs, ϱ = 1.75 g/cm 3), H 5 = 50 ka (MS = 53 μcgs, ϱ = 1.75 g/cm 3), H 6 = 64 ka (MS = 100 μcgs, ϱ = 1.69 g/cm 3). Heinrich events at Rockall Plateau sites point to a northward flow of icebergs in the northeastern Atlantic which may indicate a flow pattern to regions north of 54 °N.

Astronomical time-scale for physical property records from Quaternary sediments of the northern North Atlantic

Deep sea sediment cores taken between 50° and 75°N in the North Atlantic, in water depths varying between 1340 and 3850 m, were examined to provide an astronomically calibrated late Quaternary time-scale based on physical property records. Magnetic susceptibility and gamma ray attenuation porosity evaluator (GRAPE) density changes of these cores revealed significant responses to orbital forcing in the eccentricity (100 kyr), obliquity (41 kyr) and precessional (23, 19 kyr) bands. At 75°N (Greenland Sea), a response to obliquity forcing was weak despite the fact that it should become more pronounced in sediments at high latitudes. Application of bandpass filtering at the obliquity period (41 kyr), however, showed that variance at this period did exist in the magnetic susceptibility record, but at a very low power. At 50°N stacked curves of magnetic susceptibility correlated strongly with the SPECMAP curve for the past 500 ka. Since about 65 ka, dropstone layers are recorded in both magnetic susceptibility and GRAPE data of Rockall Plateau sediments. Although Rockall Plateau sediments show peaks in physical properties that correlate with Heinrich events (H1, H2, H4, H5, H6), such a relationship was not readily observed in Norwegian-Greenland Sea records. Heinrich events at Rockall Plateau sites indicate a northward flow of icebergs in the eastern North Atlantic. This flow pattern and the presence of Heinrich events during the past 65 ka raise the questions of whether similar events occurred before this time period, and to what kind of ice sheet dynamics and climatic-oceanographic conditions favoured major iceberg surges from the Laurentide ice sheet to the North Atlantic at 50°N.

Long Climatic Time Series from Sediment Physical Property Measurements

ABSTRACT With an accurate understanding of the lithologic dependence of pre-compaction porosity in pelagic sediments it should be possible to calculate mass accumulation rate histories of sedimentary phases, to generate proxy carbonate logs from core density scans and from seismic profiles, and to reconstruct the mechanical and chemical processes that occur during burial and lithification. We demonstrate that routinely made continuous Gamma Ray Attenuation Porosity Evaluator (GRAPE) wet bulk-density measurements of sediments show a positive relationship to carbonate content in a variety of pelagic environments. Mineralogically related differences in packing, rather than differences in grain density, appear to control wet bulk-density variations. A hyperbolic model calibrated at DSDP/ODP drill ites quantifies the relationship of density to carbonate content in different pelagic environments. The curvilinear relationships appear to converge to a similar wet bulk density (1.71 ± 0.07 g/cm3, porosity = 58%) at 100% carbonate content, but they exhibit differing porosity at zero carbonate content. Wet bulk-densities for a given carbonate content depend on the non-carbonate mineralogy and grain size, and increase systematically from opal- to clay- to quartz/lithic fragment-dominated settings. Because contrasts in sediment wet bulk-density control the seismic response of unlithified sediments, it should be possible to generate carbonate profiles remotely in carbonate-rich settings after core calibration of the seismic system. Our model can generate long-proxy carbonate curves with a typical sample spacing of 0.6-1 cm from existing GRAPE data bases in equatorial Pacific, equatorial Atlantic, and northern Atlantic locations over the past 6 m.y. These time series are used to investigate the long term evolution of climate in the Milankovitch frequency band, as well as to reconstruct patterns of carbonate sedimentation over time. Sedimentation bears the imprint of Milankovitch cyclicity superimposed on long term trends. The switch in carbonate sedimentation from 41 kyr periodicity during the Matuyama chron to dominant 100 kyr periodicity in Brunhes chron previously described in the North Atlantic occurs in equatorial Atlantic and Pacific locations as well. The lithological dependency of porosity in uncompacted pelagic sediments also has important implications for understanding later compaction and modification of pelagic sediments. In particular, the GRAPE observations suggest that differential burrow flattening observed in lithified limestone-marl alternations results from mechanical and pressure solution compaction of strata that had strong primary porosity differences, rather than from diagenetic unmixing of carbonate.