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SCAR Report No 16,

Appendix 7

MESOZOIC-PALEOGENE PALEOCEANOGRAPHY AND MARINE BIOSPHERE: KEY EVENTS
Sherwood W. Wise, Jr., Dept. of Geology, Florida State University and
The Antarctic Marine Geology Research Facility, Tallahassee, Fl 32306-3026

A synopsis of key Mesozoic-Paleogene global and regional paleoenvironmental events that help delineate the evolution of the Southern Ocean was given recently by Harwood and Wise (1995), therefore this report will update or expand upon portions of their compilation with suggestions on how future drilling and exploration can address the many associated unsolved problems. For a recent comprehensive review of Antarctic paleoenvironments through the Cenozoic, the reader is referred to Barrett (1996).

The Development of incipient Mesozoic basins during the breakup and dispersal of southern Gondwana continents:

At present the no detailed paleoenvironmental history can be written of the early breakup history and the development of the Late Jurassic-Early Cretaceous anoxic "black shale" basins, although the rough outline of the tectonic history can be recon-structed via plate tectonics (see Eliot, this symposium). We do know from drilling along the Dronning Maud Land margin at ODP Sites 692 and 693 that total organic contents reached 18.4% by the Valangenian then decreased to about 2.5% by the early Albian, but much of that Lower Cretaceous section in that region remains to be drilled. Final ventilation of the basin occurred during the Albian and culminated with upwelling and enhanced productivity that produced a siliceous ooze of exquisitely well preserved diatoms, silicoflagellates, and radiolarians, many of which are new to science.

Problem: Not known is how and in what stages this incipient Weddell Sea basin came to be ventilated. Mutterlose and Wise (1990) could only speculate on how the opening of critical gateways may have led to the ventilation of this and adjacent basins. More importantly, no model for the development of basin anoxia can be agreed on for lack of deep water as well as shallow water drill sites (existing sites record paleo water depths of only 200 to 500 m). Thus it is impossible to invoke with confidence either a restricted basin (Zimmerman et al., 1987) or an high productivity, upwelling model (Farquharson, 1983).

Approach: Clearly, the remaining section through the entire Lower Cretaceous "black shale" sequence needs to be drilled out on the Dronning Maud Land margin (i.e., the interval between those recovered at ODP Sites 693 and 693). The section should be continued into and ideally through the Jurassic section to the Explora Wedge dipping reflectors beneath. This will provide for this region a ventilation history of the basin and correlations to possible gateway events can be established. Secondly, equiva-lent strata need to be drilled on the floor of the adjacent Weddell Sea to provide deep water sites required to constrain the models of basin stagnation mentioned above.

Feasibility: As with many of the problems discussed at this workshop, regional seismic studies are now far ahead of the drill bit when it comes to exploring the Jurassic-Lower Cretaceous of the Weddell Sea margins. As Jokat (this workshop) has outlined so thoroughly in his contribution, near surface outcrops of older strata are so accessible along this margin that both shallow and deep pene-tration drilling can be employed effectively in retrieving a de-tailed record of the early basin history. Of particular interest are the near surface occurrences of older strata over Polar Stern Bank on the floor of the basin off Dronning Maud Land. These could provide the needed deep basin comparisons with the shelf/slope sites.

ODP proposal #503 has been submitted to address the deep sea drilling aspect of this problem. The shallow-water drilling technology needed for broader coverage should ideally include a dynamically positioned vessel and a drilling rig with heave compensation to deal with the deep Antarctic shelves and any ocean swells that develop.

Late Cretaceous History:

At present there are virtually no Upper Cretaceous marine drill cores from the Antarctic continent. The most complete section in the region is provided by the well-studied DSDP Sites 327 and 511 on the Falkland Plateau, which provide a near contin-uous record from the upper Albian to Maastrichtian, and for which a stable isotope record has recently been developed (Huber et al, 1995; Huber and Hodell, 1996). The lack of such a record in the southern Weddell Sea area is attributed to a pervasive regional disconformity, above which lies lower Oligocene sediment at ODP Site 693. Not enough information is known from most other re-gions around the continent to even speculate on the existence of Upper Cretaceous sediments outside the Weddell Sea and Seymour Island. O'brien and Leitchenkov (this workshop) note seismic evidence for a significant thickness of undrilled sediments between the Paleogene and Lower Cretaceous in Prydz Bay; whether these are marine or non-marine is not known.
In addition the Falkland Plateau sequence, excellent carbon-ate Campanian-Maastrichtian sections have been drilled on Maud Rise and the Kerguelen Plateau. Middle Campanian sediments are missing, however, at all Southern Ocean DSDP/ODP sites drilled to date, apparently as a result of deep sea dissolution or strong current erosion associated with circulation changes at this time, which also witnessed the development of a Late Cretaceous cool-water, provincial austral planktonic calcareous fauna and flora. Huber and Watkins (1993) attribute this to the development of the first semblance of a "proto circum-Antarctic current", perhaps in response to the opening of a gateway between Australia and Ant-arctica and/or a rise of sea level. Why this circulation system apparently closed down near the end of the Cretaceous is not understood.

Problem #1. There is virtually no Upper Cretaceous record from the Antarctic margin proper (as indicated above), hence no geo-logic history for this interval of time.

Problem #2. There is no way to study the evolutionary pathways between the primitive diatom and silicoflagellate assemblages discovered at Site 693 and the more highly evolved high southern latitude Campanian-Maastrichtian assemblages from other sites in the region (e.g., DSDP Site 275, Campbell Plateau).

Problem #3. Because no deep sea sections record the establishment of the Late Cretaceous "proto circum-Antarctic current", expanded shallow water sections along the margins of Antarctica must be sought where dissolution and strong ocean currents may not have destroyed the record.

Approach:

A reconnaissance mode of deep sea drilling will be necessary to find Upper Cretaceous sections along the margins of the Antarctic continent. Most sites proposed today through the ODP panel complex are based on highly refined seismic and ground truth information, and a high degree of certainty usually exists as to what will be drilled. A high degree of uncertainty and risk-taking must be accepted for sites proposed along the margins of Antarctica. Similarly, our state of ignorance is such that the role of serendipity must not only be expected but exploited as a means of making major break throughs in knowledge. Drilling plans and objectives should not be so highly structured that they can't be easily modified to take advantage of unexpected discov-eries, even if major changes in mission and objectives might have to be made at mid cruise. History has shown repeatedly that crustal/sedimentary models and seismic predictions can only be taken as a rough guides as to what will be encountered during reconnaissance drilling (e.g., ODP Sites 692 and 693; Barker, Kennett, et al., 1988). Unexpected surprises that provide highly valuable new information occur even in much better studied areas (e.g., ODP Site 1069, Iberia Abyssal Plain; Whitmarsh, Beslier, Wallace, et al., 1998, in press). Frontier areas must be ap-proached with great flexibility and an open mind.

K\T Boundary Crisis:

Although many accept the hypothesis that the discovery of the Chicxulub structure in Mexico has provided the "smoking gun" behind the K\T biotic crisis, there is still speculation and contention as to the effect of that crisis on distant Antarctica, which is far removed from the meridional wind belts that would have circulated debris from the event around the globe. Although an iridium anomaly was detected at Site 690 on Maud Rise and on Seymour Island, no associated tektites or shocked quartz have been found at these localities or elsewhere around the Antarctic continent.

Problem #1: Based on a study of nannofossils across the K/T sections throughout the Southern Ocean and beyond, Pospichal (1994) concluded that there was no diminished response among calcareous nannoplankton to the K/T event at high latitudes. This stands in conflict with the conclusions of colleagues who have looked at other fossil groups in sequences on the Antarctic Peninsula (e.g., Zinsmeister et al., 1989; Keller, et al., 1993).

Problem #2: There is also disagreement over the mode and tempo of the extinctions. Do some Cretaceous forms traditionally thought to have gone extinct abruptly at the boundary actually survive the event and persist into the Tertiary to undergo a series of step-wise extinctions as suggested by Canudo et al. (1991)? A somewhat similar case has been made for ammonites of the Antarctic Peninsula by Zinsmeister, et al. (1989). Indeed, Elliot et al. (1994, p. 678) state that "The Seymour Island provides no compelling evidence for mass extinction at the K-T boundary", which seems to be true for the fossil groups they studied (dino-flagellate cysts and invertebrate macrofossils; calcareous micro-fossils, which many think were more sensitive to the event that siliceous or cellulose-walled groups, are not preserved in the boundary beds). Perhaps if there was a refugia from the K/T devastation seen elsewhere around the globe, Antarctica might be the place to look for it. Outside of Seymour Island, Site 690 on Maud Rise, Site 738 on the Kerguelen Plateau and Site 752 on Broken Ridge, however, no other stratigraphically complete K/T boundary sections are known from the Southern Ocean region, and none beyond Seymour Island have been reported from the continent itself.

Approach:

As with the Albian diatomite discussed in the previous section, seismic stratigraphy cannot resolve intervals as thin as the K/T boundary, much less detect a complete boundary as opposed to an incomplete one. Thus a reconnaissance mode of exploration as discussed above will be necessary to add to our knowledge of this event in Antarctica. Expanded nearshore clastic sections have the advantage of allowing the event to be studied at a higher stratigraphic resolution, but such sections also dilute the content of open marine microfossils and can make age control more difficult.
The Paleocene-Eocene boundary thermal maximum and benthic foram extinction event; the reversal of latitudinal global circulation patterns:
This event, as well as hypotheses as to its significance and impact on global climate, was defined from the study of ODP Leg 113 cores from Maud Rise. In addition to recording the most massive extinction of benthic foraminifers in the last 80 m.y., it is marked by a carbon isotope excursion and an sharp increase in the percentage of warm-water loving discoasters. It has since been recognized in shelf sediments of the New Jersey coastal plain and in terrestrial sediments of the Paris Basin (Stott et a., 1993). A popular hypothesis attributes it to a short-term increase in atmospheric C02 as a result of excessive volcanism, presumably during the rifting of the Norwegian Sea or elsewhere, such as in the Carribean Sea.

Problem #1: It has been suggested that the event was accompanied by a reversal in the latitudinal circulation pattern of the oceans (Kennett and Stott, 1990, a suggestion that has engendered controversy and the need for further research.

Problem #2: The Paleocene record leading up to the event is cyclic, showing alternations in clay and carbonate content, with much of the clay being kaolinite derived from the Antarctic continent. Additional sequences of this nature are needed to provided a detailed climate history of Paleogene Antarctica prior to glaciation.

Approach:

Kennett and Stott (1990) and subsequent workers based their findings on two drill holes, one near the crest and one near the base of Maud Rise. More rigorous constraints could be placed on their various models by completing a depth transect of sites down the flank of the Maud Rise so that a greater range of the paleo water mass(s) could be sampled, correlated and com-pared. In addition, the discovery of this short term (20 k.y.) event on the margins of Antarctica would provide not only the highest latitude record known, but also would show its effects on high latitude, shallow water fauna and flora.
Feasibility: APC coring on Maud Rise would be a relatively simple procedure because of the availability of good seismic data at the necessary water depths. This is an objective of ODP Proposal #530. Discovering evidence for the event on Antarctica would require a most aggressive program of drilling for seeking out and drilling Paleogene sediments.

Evolution of Southern Ocean and Antarctic Biota:

Much has been learned from DSDP/ODP about biostratigraphic correlations throughout the Southern Ocean region, where rela-tively cosmopolitan calcareous faunas and floras from the world ocean interface with the more endemic, siliceous high-latitude faunas and floras that vary more widely with latitude and water conditions. The siliceous groups displaced the calcareous forms northwards as climate cooled during the Oligocene to Recent, emerging as the mainstays of biostratigraphic correlations for the Neogene. Efforts are being made to integrate diatom and nannofossils stratigraphies throughout the region in order to enhance their utility (Baldauf and Ramsey, in preparation). Exploration of new areas around the continent, however, will inevitably lead to further modification and refinement of zonal schemes.
Problems: Virtually nothing is known about the effects of conti-nental connections, barriers or seaways that may have influenced the dispersal capacity and adaptive stress of species as might be indicated by their endemism, provincialism, diversity and evolu-tionary rates. As noted by Harwood and Wise (1995), "...The degree that mechanisms such as basin rifting, surface water cooling, oceanic turnover, etc. influenced the paleobiogeography, evolution and morphological variation of Southern Ocean and Antarctic faunas is unknown.

Approach:

Extensive and aggressive drilling, including drilling into the sub-ice sedimentary basins of Antarctica as called for by Webb (this Workshop), will be necessary before such complex issues as those stated above can possibility be addressed.
The inception of glacio-marine sedimentation during the Cenozoic (Eocene?) and transition from greenhouse to icehouse climates:
Of all the problems that have driven scientific exploration of Antarctica over the past 30 years, obtaining a record of the pre-glacial/glacial transition has been the goal sought most ardently but with the least amount of success. This has become the "Holy Grail" of many Antarctic crusades. Harwood and Wise (1995) outline previous attempts by deep sea drilling to obtain this record, all of which failed. The Cape Roberts Project will be the next major attempt in this endeavor.
Wise et al. (1991, 1992) summarized occurrences of known or suspected Paleogene Ice Rafted Debris, noting several reported examples from the early to middle Eocene, none of which were well accepted due to dating problems or suspected contamination. Nonetheless, these have remained as enigmas that could not be summarily dismissed despite the lack of supporting climatic evidence or other indicators of glacial activity during this relatively warm period in Earth history. Among these are the set of four Eltanin cores from the Southeast Pacific in which Margo-lis and Kennett (1971) found unmistakable ice-rafted sand grains together with lower to middle Eocene nannofossils and planktonic foraminifers of warm-water affinities. This paradox has appar-ently been resolved by Gersonde et al.'s (1996) discovery that the cores lie in the vicinity of a Pliocene extraterrestrial impact site, and that Neogene IRD has been mixed with the older materials.
Another middle Eocene date for glacial activity from the base of the CIROS-I core (Hambrey and Barrett, 1993; Hannah, 1994) has been reinterpreted as latest Eocene on the basis of a recent paleomagnetic study (Wilson, et al, 1998); reworked microfos-sils are suspected for the older date. Reported upper Eocene IRD from the Kerguelen Plateau (Ehrmann, 1991) has been shown to be down-core contamination (Breza and Wise, unpublished data). Thus, most reported direct evidence of Eocene Antarctic glacia-tion have come under challenge except for the late Eocene date for the CIROS I core as well as for ODP Sites 739 and 742 in Prydz Bay, which are based primarily on paleomagnetic reversal patterns. Neither of these cores are thought to have penetrated completely through glacial into preglacial marine sediments, therefore the pre-glacial\glacial sediments transition has yet to be documented.
The elimination of early to middle Eocene candidates for Antarctic glaciation is more in tune with independent paleocli-mate evidence, including important new findings from the Antarctic continent, which is being compiled from erratics collected near McMurdo station. These include "an excellent paleo fauna and flora, including crocodiles, sharks teeth, and pelicanformes bird bones" best dated from microfossils as late mid Eocene to lower upper Eocene (D. Harwood, 1997, pers. comm.; these new studies are to be published in the Antarctic Research Series).
A consensus has developed within the community, however, that major glaciation began in the earliest Oligocene, and that "there has been at least some ice on that continent since that time" (Kennett and Barron, 1992, p. 9). The details of the subsequent history have yet to be revealed, however, and remains the subject of future research. This has been the subject of several reports in this workshop, and won't be further discussed here except to note that the same impact event associated with the Eocene Eltanin cores mentioned above might also be responsi-ble for the emplacement Pliocene and older microfossils into the Sirius Formation, the origins of which have long been a subject of strong debate. An assessment of the impact of this new evi-dence on the Sirius problem will be given by D. Harwood at the symposium on Antarctic climate change that follows this workshop.

References

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