Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene

Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene

Determination of and preliminary results from the high-resolution physical properties record of the AND-1B sediment core from beneath Ross Ice Shelf, Antarctica

A more than 1200 m long sediment core was drilled beneath McMurdo Ice Shelf near Ross Island (Antarctica) in austral summer 2006/07 (ANDRILL-MIS Project). High-resolution whole-core physical properties were determined as one set of parameters to describe changes in the depositional system over the sedimentation period of about 12 myrs incorporated in the core. Four parameters were measured using a multi-sensor core logger: acoustic velocity, wet-bulk density, non-contact electrical resistivity and magnetic susceptibility. Data quality was routinely controlled by measurement of standards. Deviations from the reference values are minimal with regard to the whole spectrum of sediment data points and no offsets between core diameter intervals are obvious. Almost all boundaries between lithostratigraphic units are in good agreement with changes in the physical properties record. For the depth interval between 140-300 mbsf the physical properties indicate rhythmic changes in the environmental system with alternations of diatomite and diamictite sequences

Refined porosities of the AND-1B core reflecting the glacial history in the McMurdo region

During austral summer 2006/07 the ANDRILL program successfully drilled the 1285 meters long AND-1B sediment core from beneath the McMurdo Ice Shelf covering the last 12 Ma. One of the major goals is to better constrain the past Ross Ice Shelf/ice sheet history. High-resolution measurements of whole-core wet-bulk densities (WBD) were collected at the drill site laboratory using a Multi-Sensor Core Logger. WBD can be applied to calculate fractional porosity if the grain density (GD) is known. GD values of about 1,200 discrete samples were determined through pycnometer measurements, ranging from 2.15 to 2.9 g/ccm. A mathematical relationship was established between WBD and GD in order to calculate porosity from the entire WBD data. The overall down-core trend of the porosity holds information about the compaction history of the deposited sediments. Removing this trend allows for detailed studies of the residual porosities in terms of regional environmental change related to ice shelf/ice sheet dynamics, in particular overconsolidation through ice-loading. The down-core compaction-trend anaysis is based on density changes for shales and siltstones. Overconsolidation is mainly found in diamictite units and here significantly more pronounced in the upper 200m indicative of intensive ice-sheet grounding during the last 2 Ma

ANDRILL-MIS: Einsicht in den Bereich Physical Properties

Im Rahmen des internationalen ANDRILL-Projektes wurde im australischen Sommer 2006/07 das erste Bohrprojekt (MIS McMurdo Ice Shelf Project) seiner Art durchgeführt, bei dem ein mehr als 1,2 km langer Sedimentkern vom antarktischen Schelf nahe Ross Island, McMurdo Sound, abgeteuft wurde. Der Bereich Physical Properties ist einer von vielen zur Kerncharakterisierung in diesem multidisziplinären Vorhaben. Direkt an der Bohrstelle wurden mittels MSCL-Messbank (Multi-sensor core logger) und anschließendem Prozessieren die folgenden vier physikalischen Eigenschaften des Sedimentkerns bestimmt: Feuchtdichte, akustische Geschwindigkeit, magnetische Suszeptibilität sowie die elektrische Leitfähigkeit. Geloggt wurde dabei in Runs von 3-6 m Länge mit einzelnen Kernsektionen von jeweils 1 m Länge. Je nach Kernnachschub wurde in Intervallen von 1-4 cm gemessen, was eine hochauflösende Betrachtung der einzelnen Parameter ermöglicht. Im Vorfeld der Bohrung wurden anhand von seismischen Profilen mehrere Reflektorhorizonte identifiziert und der Bilious Green-Reflektor in etwa 1200 mbsf Tiefe zum target reflector der Bohrung erklärt. Die Bestimmung der akustischen Geschwindigkeit an der Bohrstelle diente in diesem Zusammenhang nicht nur der Charakterisierung des Kerns an sich, sondern auch zur Kontrolle über das Erreichen des target reflector. Geschwindigkeiten und Porositäten über das Gesamtprofil des Kerns zeigen einen deutlichen Trend, wobei erstere zunehmen von etwa 1,2 km/s auf 3 km/s und letztere abnehmen von etwa 0,7 auf 0,3. Die obere Hälfte des Kerns bis ca. 600 mbsf zeigt deutliche Abweichungen von einer fiktiven Trendlinie in beide Richtungen, während die unteren 600 m im Vergleich dazu weniger Variation aufweist. Insbesondere in den zwei Parametern Dichte und Suszeptibilität lassen sich in den oberen 100-600 m synchrone Rhythmen ausmachen, die auf einem Fazieswechsel zwischen Diatomiten und Diamikten basieren. Um die Zyklizität genau zu bestimmen, muss das vorläufige Altersmodell präzisiert und in diesem Zusammenhang die Bedeutung der glaziogenen Erosionsdiskordanzen geklärt werden. Die weitere Arbeit sieht Pyknometer-Messungen zur Bestimmung einer genaueren Korndichte zur Porositätsberechnung vor, die Berechnung des Kompaktionstrends anhand von Vergleichen derselben Lithologie aus verschiedenen Tiefen und eine detaillierte Untersuchung zementierter Horizonte

The physical properties record of the AND-1B sediment core A quantitative approach to describing past ice shelf/ice sheet dynamics

In austral summer 2006/07 a more than 1200 m long sediment core was drilled beneath McMurdo Ice Shelf near Ross Island (Antarctica) with the purpose of contributing to a better understanding of the Late Cenozoic history of the Antarctic Ice Sheet (ANDRILL-MIS Project). Dating back to about 13 Ma this core offers the great potential to study the long-term global cooling trend at an ice-proximal location. High-resolution multi-sensor core logger measurements of whole-core physical properties serve as a numerical expression of lithologic changes in the core and therefore, represent a quantitative tool for approaching past ice dynamics. This is especially applicable for the repeating sequences of diatomites and diamtictites in the upper half of the core with a prominent cyclicity between 140-300 mbsf. Rather abrupt high-amplitude variations in wet-bulk density (WBD) and magnetic susceptibility (MS) reflect rapid changes between two main end-member facies generated by the alternation between a grounded ice sheet and open marine conditions. For the whole core, the WBD signal, ranging from 1.4 kg/cu.m in the diatomites to 2.3 kg/cu.m in diamitctites from the lower part of the core, represents the influence of three variables: (i) the degree of compaction seen as reduced porosities with depth from about 55 % in the top part to about 25 % at the bottom, (ii) the clast content with clasts being almost absent in diatomite deposits and (iii) the individual grain density (GD). GD itself ranges from about 2.15 kg/cu.m in diatomites to 2.9 kg/cu.m for volcanic sandstones and thereby reflects the variety of lithologies as well as the influence of cement (mainly pyrite and carbonate) on the matrix grain density. The calculation of residual porosities demonstrates the strong imprint of glacial loading for especially diamictites from the upper 150 m. This over-consolidation signature preserved in the sediments can be used to characterize the Pleistocene ice sheet in terms of ice thickness derived from past stresses. MS on the other hand mainly documents a marine vs. terrestrial source of sediments where the latter can be divided into younger local material from the McMurdo Volcanic Province and basement clasts from the Transantarctic Mountains. Values range over several orders of magnitude from <10 (10-5 SI) in the diatomites to 8000 (10-5 SI) in single clasts (mainly dolerite). Synchronous minima and maxima in both WBD and MS support dramatic changes in the depositional environment, driven by oscillations in ice extent in response to climate fluctuations. According to the age model, cyclicity occurs on Milankovich timescales with changes in obliquity being the dominant forcing until the Early Pleistocene. Additionally, small-amplitude variations within diatomite units propose sub-Milankovich forcing as superimposed control on system dynamics

Heat Flow and Hydrologic Characteristics at the AND-1B borehole, ANDRILL McMurdo Ice Shelf Project, Antarctica

The Antarctic Drilling Program (ANDRILL) successfully drilled and cored a borehole, AND-1B, beneath the McMurdo Ice Shelf and into a flexural moat basin that surrounds Ross Island. Total drilling depth reached 1285 m below sea floor (mbsf) with 98 percent core recovery for the detailed study of glacier dynamics. With the goal of obtaining complementary information regarding heat fl ow and permeability, which is vital to understanding the nature of marine hydrogeologic systems, a succession of three temperature logs was recorded over a five day span to monitor the gradual thermal recovery toward equilibrium conditions. These data were extrapolated to true, undisturbed temperatures, and they define a linear geothermal gradient of 76.7 K/km from the seafloor to 647 mbsf. Bulk thermal conductivities of the sedimentary rocks were derived from empirical mixing models and density measurements performed on core, and an average value of 1.5 W/mK ± 10 percent was determined. The corresponding estimate of heat fl ow at this site is 115 mW/m2. This value is relatively high but is consistent with other elevated heat-fl ow data associated with the Erebus Volcanic Province. Information regarding the origin and frequency of pathways for subsurface fluid flow is gleaned from drillers’ records, complementary geophysical logs, and core descriptions. Only two prominent permeable zones are identified and these correspond to two markedly different features within the rift basin; one is a distinct lithostratigraphic subunit consisting of a thin lava fl ow and the other is a heavily fractured interval within a single thick subunit