Subsidence, Tilting, Sedimentation, and Oligocene-middle Miocene paleo-depth of Ross Sea

Introduction

The Ross Embayment, including Ross Sea, Antarctica, was a high-elevation plateau that has been subsiding since 104 million years ago (Ma) due to crustal stretching and thinning (Luyendyk et al., 2001; Bialis et al., 2007). This subsidence to the current water depths is important because elevation or water depth affect whether ice sheets will grow and advance at given environmental conditions such as CO2 level and ocean water temperatures.

We completed a seismic stratigraphic interpretation for most of the 900 km (E-W) by up to 600 km (N-S) Ross Sea. Interpreted stratigraphic horizons include six inferred early Oligocene (or older), and dated late Oligocene through Middle Miocene unconformities and other reflectors; the interpretations of the reflections were gridded. Where these reflectors are not present because of outcrop at the seafloor, onlap onto acoustic basement, or truncation in the sub-bottom by a young composite unconformity (“Red”), the grids were respectively merged with trimmed grids of sea floor, acoustic basement, and the Red unconformity. A 3D velocity model was constructed for the western half of Ross Sea in order to convert the six horizons grids from two-way travel time to depth. The velocity model was based on sonobuoys and selected stacking velocities from multichannel seismic reflection. One of these horizons is the sea floor.

Backstripping and thermal modeling is commonly done in order to understand the paleo-elevation or water depth at times in the past (De Santis et al., 1999; Decesari et al., 2007). Our depth-converted horizon grids can be used by others for regional tectonic subsidence modeling, and to restore erosion, as was done by Wilson and Luyendyk (2009) using the ANTOSTRAT (1995) digital maps (also, Wilson et al., 2012). Paleotopography is a critical input to Global Climate Modeling for past ice sheets (e.g., Wilson et al., 2013). We have produced preliminary paleo-depth grids. Modified paleo-depth grids will be made available at a later time.

Intellectual Merit (and Results)
Terror Rift 23-13 Ma Passive Subsidence vs. Ongoing Extension
The part of Terror Rift north of Ross Island is almost 400 km-long. Several publications propose that southern Terror Rift did not experience active extension and crustal thinning between 23 and 13 Ma (example: Fielding et al., 2008). In these interpretations, there was renewed rifting after 13 Ma. Our interpretations indicate active faulting across the east side of northern Terror Rift after ~21 Ma, and possibly before then. Furthermore, our interpretation does not support significant change in subsidence rates of the deepest part of Terror Rift basin after 29 Ma.

Age of an Unconformity and Processes Responsible for it.
We investigated whether glaciers eroded Ross Sea soon after 34 Ma. Different members of our Italian-American research group prefer different processes for the last erosion of the unconformity, and the time when this occurred. Possibilities include subaerial erosion in an arid rift basin ending about 55 Ma, submarine canyons or rivers last eroding soon after 55 Ma, and erosion by ice soon after 34 Ma (Sorlien et al., 2016). Extrapolation of the sedimentation rate between the 25 Ma and 29+ Ma unconformities is permissive of top synrift being 34 Ma. However, the paleo-depth of top-syn-rift is 2.5 km for the deepest part of Central Trough if 34 Ma. This may be too deep for grounded ice given the CO2 levels and ocean water temperatures at this time. However, ice may have eroded less deep flanks of this trough.

Paleo-depth of Ross Sea
Douglas Wilson performed paleo-depth reconstructions for several unconformities, including ~15 Ma, ~21 Ma, ~25 Ma, ~29 Ma +/-, and top syn-rift/acoustic basement. The resulting digital maps indicate that much of Ross Sea was close to sea level at 25, 21, and 15 Ma. It seems logical that a Global Climate Model using these reconstructions might predict ice sheets advancing and retreating across a shallow Ross Sea after 25 Ma.

We have more direct evidence that grounded ice advanced north of the current edge of the Ross Ice Shelf into Eastern Basin of Ross Sea at ~25 Ma. A stack of unconformities is imaged, with the top one, the Blue “Mid-270” ~25 Ma unconformity, cutting deeper eastward towards the center of Eastern Basin. The paleodepth map for this unconformity has the area of this seismic reflection profile near sea level.

Broader Impacts
The international collaboration with Instituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) in Italy, continues. An Italian Ph.D. student is part of this collaboration, as is a University of North Carolina Ph.D. student who completed his degree. These student projects respectively model 14 Ma paleo-topography of Ross Sea (Olivo et al., 2017), and analyze glacial marine stratigraphy dated between 21 and 29 Ma beneath eastern Ross Sea (Brazell, 2017). An undergraduate also was supported by this project; she participated in the SURFO program at the University of Rhode Island Graduate School of Oceanography (Davis et al., 2014).

Other researchers can model paleo-topography of this part of Ross Embayment using the depth grids, or they can use our paleodepth grids as they become available. Paleodepth/paleoelevation grids are used in Global Climate Models to understand ice sheet growth and retreat.

REFERENCES

ANTOSTRAT (1995), Seismic Stratigraphic Atlas of the Ross Sea, in Geology and Seismic Stratigraphy of the Antarctic Margin, edited by A. K. Cooper, Barker, P. F., Brancolini, G., p. 22 plates, American Geophysical Union, Washington, D.C.

Bialas, R. W., W. R. Buck, M. Studinger, and P. G. Fitzgerald (2007), Plateau collapse model for the Transantarctic Mountains–West Antarctic rift system; insights from numerical experiments, Geology, 35, 687–690.

Brazell, S. J., 2017, Seismic Record of West Antarctic Ice Sheet Dynamics During the Late Oligocene to Early Miocene in the Eastern Basin, Ross Sea, Ph.D Dissertation, Department of Geological Sciences, University of North Carolina at Chapel Hill, 100 pages.

Davis, S., C. Sorlien, L. De Santis, B. Luyendyk, C. Sauli, N. Wardell, P. Bart, (2014), Record of earliest West Antarctic ice sheet beneath Ross Sea?, 2014 Fall Meeting, AGU: San Francisco, Calif., Abstract PP21A-1302 presented at 2014 Fall Meeting, AGU, San Francisco, Calif., 15-19 Dec.

De Santis, L., S. Prato, G. Brancolini, M. Lovo, and L. Torelli (1999), The Eastern Ross Sea continental shelf during the Cenozoic: Implications for the West Antarctic ice sheet development, Global and Planetary Change, 23, 173-196.

Decesari, R. C., D. S. Wilson, B. P. Luyendyk, and M. Faulkner (2007), Cretaceous and Tertiary extension throughout the Ross Sea, Antarctica, in Antarctica: A Keystone in a Changing World – Online Proceedings of the 10th ISAES X, edited by A. K. Cooper, C. R. Raymond et al., pp. DOI: 10.3133/of2007-1047.srp3098.

Fielding, C.R., Whittaker, J., Henrys, S.A., Wilson, T.J., Naish, T.R., 2008, Seismic facies and stratigraphy of the Cenozoic succession in McMurdo Sound, Antarctica: implications for tectonic, climatic and glacial history. Palaeogeography, Palaeoclimatology, Palaeoecology 260, 8–29, doi:10.1016/j.palaeo.2007.08.016.

Luyendyk, B. P., C. C. Sorlien, D. S. Wilson, L. R. Bartek, and C. H. Siddoway (2001), Structural and tectonic evolution of the Ross Sea rift in the Cape Colbeck region, Eastern Ross Sea, Antarctica, Tectonics, 20, 933-958.

Olivo, E., L. De Santis, N. Wardell, R. Geletti, M. Busetti, C, Sauli, A. Bergamasco, F. Colleoni, W. Vanzella, C. Sorlien, D. Wilson, R. DeConto, R. Powell, P. Bart, B. Luyendyk, 2017, Paleobathymetric Reconstruction of Ross Sea: seismic data processing and regional reflectors mapping, Geophysical Research Abstracts 19, EGU2017-8737, Session CL1.31, EGU General Assembly, Vienna, Austria

Sorlien, C. C., C. Sauli, L. De Santis, B. P. Luyendyk, N. Wardell, S. M. Davis, D. S. Wilson, S. Brazell, L. Bartek, P. J. Bart, 2016, Seismic stratigraphic interpretations suggest that sectors of the central and western Ross Sea were near and above sea level during earliest Oligocene time, Abstract C52B-08 presented at 2016 Fall Meeting, AGU, San Francisco, Calif., 11-16 Dec.

Wilson, D. S., and B. P. Luyendyk (2009), West Antarctic paleotopography estimated at the Eocene-Oligocene climate transition, Geophysical Research Letters, 36(L16302), 1-4.

Wilson, D. S., S. S. Jamieson, P. J. Barrett, G. Leitchenkov, K. Gohl, and R. D. Larter (2012). Antarctic topography at the Eocene–Oligocene boundary. Palaeogeography, Palaeoclimatology, Palaeoecology, 335-336, 24-34.

Wilson, D. S., D. Pollard, R. M. Deconto, S. S. Jamieson, and B. P. Luyendyk (2013), Initiation of the West Antarctic Ice Sheet and estimates of total Antarctic ice volume in the earliest Oligocene, Geophysical Research Letters, 40(16), 4305-4309.