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SCAR Report No 16,

Appendix 7

GLACIER-SEA INTERACTIONS AND THE UTILITY OF PROCESS STUDIES FOR
INTERPRETING THE ANTARCTIC GLACIAL AND CLIMATIC RECORD ON DIFFERENT TIME SCALES
Ross D. Powell,
Dept. of Geology, Northern Illinois University,
DeKalb, IL 60115, U.S.A.

Glacial and climatic records are best inferred by using quantitative, predictive models based on well documented processes. That is true for paleo-environmental studies on any time scale and therefore the well-documented processes need to be considered in both ANTOSTRAT and ANTIME initiatives. Few Antarctic environments have been documented by comprehensive process studies for geological purposes. Thus both initiatives should involve process studies and use the data produced by them. This discussion will concentrate on processes of the marine system with minor mention of coastal and terrestrial systems.

As way of background, a review of relevant recent developments in the glacimarine system is presented. One of the prime factors to recognize for interpreting a high latitude marine record is the potential for major variability in environmental conditions and processes through time and spatially. This consideration is important for both ANTIME and ANTOSTRAT science initiatives. Studies in both initiatives need to be established with sound knowledge of possible paleo-environmental conditions and an understanding of the potential record that could be produced. Spatial variability of processes around the Antarctic ice margin appears to be significant, which is especially important for both ANTOSTRAT and ANTIME. Recognizing that 57% of the margin is floating glacier-terminus, 38% is tidewater terminus, and the remaining 5% is exposed shoreline of mostly bedrock with rare beaches. Extending this concept through time is relevant for ANTOSTRAT studies because conditions favoring ice shelf formation and maintenance (embayments, pinning points, glacial flux, equilibrium line altitudes, sea ice conditions, oceanic parameters) change with glacial advance and retreat. Under current Antarctic polar conditions, ice shelves and floating glacier tongues are most commonly fed by ice streams or fast flowing outlet glaciers. During different advances and retreats will the glaciers be continually able to feed floating termini? Considering that rates of grounding line advance and retreat are likely to differ significantly for ice stream and interstream areas and that ice streams can migrate laterally through time, lead to speculation that the glacimarine sediment record could change significantly through time at a particular study site and vary around the Antarctic margin. For example, initial glaciation on Antarctica is inferred to have been mountain (perhaps temperate?) glaciation which then developed through time to polar ice sheets. Minor glacial fluctuations since ice sheet formation are generally accepted but major changes of glacial phases are strongly debated. How and where can these issues be resolved? Do we know what to look for in the glacial record? Furthermore, climatic changes on different time-scales (e.g., millennia versus say, 100s ka) have different forcing - what is the glacier response? What is the glacial record? Does the record differ for different forcing? Ice stream and interstream areas are likely to have different terminus conditions. Do they produce different sedimentary records? How important are subglacial conditions in producing different records, both in terms of sedimentary architecture and lithofacies and biofacies?

Recent studies have made important advances in addressing some of these questions, but they have also presented their own unknowns. For example, processes forming grounding-line wedges (“till deltas”) of the Ross Ice Shelf inferred from geophysical remote sensing are yet to be verified. The inferred process of feeding sediment to the systems by subglacial till deformation is currently under debate. Furthermore, seismic reflection records on high latitude continental shelves in both hemispheres have been interpreted as displaying these grounding-line wedges (e.g., “till tongues”). Therefore, confirmation of processes of their formation is critical for establishing a reliable paleoglacial/paleoclimatic history on any time scale. In a related issue, marine geologists recently have been recognizing, especially on northern hemisphere, high latitude continental shelves, significant lateral variability in sedimentary facies along paleo-ice margins. They can be inferred to be caused by different subglacial and/or terminus conditions, but our present knowledge of such features and processes is very limited.

Verification of marine varves produced in both temperate and polar glacimarine settings is very encouraging for high resolution paleoclimatic studies if climatic controls can be linked to their forcing, as has been done in the Antarctic Peninsula. Are similar facies produced under other environmental settings? These climatic proxies have great potential for short term changes, which in ANTIME studies, can be linked to ice core records. But linkages to glacial dynamics and the consequent glacial sedimentary record still need verification. Other glacimarine records inferred to be glacially driven and being used as climatic proxies, the Heinrich layers of the North Atlantic, have recently been inferred to occur in the Southern Ocean. If correct, these findings raise numerous questions regarding processes and linkages feedbacks and lead/lag time effects, about which we now very little. How can icebergs produce extensive iceberg-rafted debris layers? What can be the teleconnections between the hemispheres? What are the glacial-climate linkages that require fast response times?

Recognition of glacimarine facies that characterize grounding line and calving line fluctuations is important especially for high resolution changes. These have been shown to vary among different glacial regimes and among terminus types due to different sedimentological processes acting. Remotely operated vehicle (ROV) studies of grounding line areas have helped in this characterization and detailed high resolution stratigraphy combined with detailed core interpretations have advanced our understanding of some aspects to look for, but many remain unknown. Integrating sedimentological and biological/paleontological data is an important step in the future.

Even some more basic aspects require attention. Absolute definition of glacial facies is still debated among geologists as is relating the facies to glaciological processes. Distinguishing among subglacial facies and between those and glacimarine facies remains also problematic although refinements continue. The meaning of variations in IBRD signatures remains unresolved and such questions as “does an apparent increase in ice rafting indicate a glacial advance or retreat, or both, or changes in sea ice cover?” remain difficult to answer. The presence and extent of sea ice beyond calving termini are also important to consider in stratigraphic interpretations. Marine geological consequences of sea ice extent and thickness still requires more evaluation.

A list of some possible future objectives for process and related studies follows.

Linked studies (both ANTIME and ANTOSTRAT)


Addressing the questions and following the suggestions outlined above for process studies in Antarctica are important steps to follow to enable significant advances in understanding both the Cretaceous-Cenozoic and late Quaternary earth history. The studies need to be concomitant with and integrated into studies of the stratigraphic record to provide reliable models and interpretations of records of past glacial and climatic changes. They are also important for providing constraining data used to build predictive models for high resolution ice and climatic changes during the late Quaternary. The data produced will also be useful for interpreting high latitude continental shelf sequences world wide. Currently, our state of knowledge of Antarctic marine glacial history falls behind many other areas in the world even with the important new results over recent years. Since initiation of IGBP’s PAGES programme, the northern hemisphere data base well exceeds that of the Antarctic for the late Quaternary. Beyond Antarctica process studies are much farther advanced. They are probably best documented for temperate settings, then subpolar settings, with many fewer studies of polar processes. If these studies are not conducted progress in integrating the Antarctic glacial record with those of other areas around the world and with global climate models (GCMs and OAGCMs) will be impacted. Reliable interpretations of stratigraphic successions will be impaired because quantitative interpretive and predictive models require modern data sets for baseline testing. The late Quaternary record for the ANTIME initiative requires process studies because they are so closely linked to the ice sheet record and its use as a climate proxy to compare with ice core records.
To achieve the goals of processes studies integrating data from different fields of science that are also closely integrated with stratigraphic studies, high levels of technology are required with instrumentally intensive, logistically complex (and/or expensive) field operations. As mentioned above, process studies of Antarctic environments relevant to interpreting the paleo-environmental record, are few. Consequently, much of the future work will be at a reconnaissance level. Field studies for data collection will require ice and sub-ice drilling, ROV work, marine and terrestrial sediment drilling, and selected long-term monitoring sites with instrumentation having smart technology to operate remotely. Circum-Antarctic studies are needed to provide a broad data base to include the natural variability in processes and to document the Cretaceous-Cenozoic and late Quaternary records.