During the Cretaceous the global climate was quite different from today: no indications for long-lived glaciation have been identified and the temperature differences between poles and equators as well as oceans and continents were much smaller. Atmospheric CO2 was much higher than today but the presently observed rise in pCO2 has led to concern that global climate may return to the ‘uninhabitable’ state of the Cretaceous. Although discussions and studies have been on-going for a number of years the trigger for the transition from the Cretaceous greenhouse to the Cenozoic icehouse has still not been identified without doubt.
A decline in atmospheric pCO2 seems to be an important driver of Cretaceous-Paleogene/Neogene climate changes in the Southern Ocean but the opening of gateways and subsequent migration of continents also has had a profound effect on the regional climate change and oceanography . Although the theory of declining CO2 levels resulting in strong cooling at the Eocene-Oligocene boundary is supported by reconstructions of Cenozoic atmospheric CO2 and modelling studies it does not answer the question why atmospheric CO2 strongly declined during the Cenozoic. Geological evidence as well as modelling studies on the other hand challenge the gateway theory. Uplift of large mountains and associated increased weathering have also been discussed to have had an influence on global climate but it is difficult to estimate how much atmospheric CO2 was removed during the process. A fourth theory discusses modified insolation patterns and seasonality which suffers from uncertainties in the used tuned age model.
Transition from the Cretaceous ocean to Cenozoic circulation
Transition from the Cretaceous ocean to Cenozoic circulation in the western South Atlantic - a twofold reconstruction
The Cretaceous oceanic circulation has been quite different from the modern with a different distribution of the continents on the globe. This has resulted in a much lower temperature gradient between poles and equator. We have studied seismic reflection data and used numerical simulations of atmosphere and ocean dynamics to identify important steps in modifications of the oceanic circulation in the South Atlantic from the Cretaceous to the Cenozoic and the major factors controlling them. Starting in the Albian we could not identify any traces of an overturning circulation for the South Atlantic although a weak proto-Antarctic Circumpolar Current (ACC) was simulated. No change in circulation was observed for the Paleocene/early Eocene South Atlantic, which indicated that this period has witnessed a circulation similar to the Albian and Cenomanian/Turonian circulation. The most drastic modifications were observed for the Eocene/Oligocene boundary and the Oligocene/early Miocene with the onset of an ACC and Atlantic meridional overturning circulation (AMOC) and hence southern sourced deep and bottom water masses in the western South Atlantic. A modern AMOC, which intensified in strength after closure of the Central American Seaway (CAS), and a strong ACC have resulted in current controlled sedimentary features and wide spread hiatusses in the South Atlantic since the middle Miocene. The opening of Drake Passage in early Oligocene times and the closure of the CAS at ~6 Ma, i.e., tectonic processes, have been identified as the key triggers for the observed most severe changes in oceanic circulation in the South Atlantic.
Drilling the Agulhas Plateau and Transkei Basin
Drilling the Agulhas Plateau and Transkei Basin to reconstruct the Cretaceous - Paleocene Tectonic and Climatic evolution of the Southern Ocean Basin
The transition from the Cretaceous “Supergreenhouse” to the Oligocene icehouse provides an opportunity to study changes in Earth system dynamics from a time when climate models suggest CO2 levels may have been as high as 3500 ppmv (parts per million by volume) and then declined to less than 560 ppmv. During the Supergreenhouse interval meridional temperature gradients were very low and oceanic deposition was punctuated by episodes of widespread anoxia, termed Oceanic Anoxic Events (OAEs) resulting in large scale burial of organic carbon reflected in positive delta 13C excursions. High CO2, greenhouse climate conditions are envisioned for the near future calling for action to get a better understanding of their potential impacts and dynamics.
Climate models have identified significant geography-related Cenozoic cooling arising from the opening of Southern Ocean gateways, pointing towards a progressive strengthening of the Antarctic Circumpolar Current as the major cause for cooler deep ocean temperatures. Analogous arguments point to an important role for deep circulation in explaining Late Cretaceous climate evolution. The Agulhas Plateau is located in a key area for retrieving high-quality geochemical records to test competing models, e.g. to what extent and exactly when the opening of Drake Passage contributed to cooling of the deep ocean. The drill sites proposed for IODP 834-Full on Agulhas Plateau and Transkei Basin are at high latitudes (65°S-58°S from 100 to 65 Ma) and within a gateway between the newly opening South Atlantic, Southern Ocean and southern Indian Ocean basins. Recovery of expanded and stratigraphically complete pelagic carbonate sequences from this region, and comparison with drilling results from Naturaliste Plateau (IODP Expedition 369), will provide a wealth of new data to significantly advance the understanding of how Cretaceous temperatures, ocean circulation, and sedimentation patterns evolved as CO2 level rose and fell, and the breakup of Gondwana progressed.
IODP Expedition 392 The Agulhas-Transkei Transect
Proposal 834-Full has now been scheduled to be drilled as Expedition 392 in Feb/March 2021. The main scientific questions and objectives to be addressed by the proposed primary and alternate drill sites (see above map) are:
(A) Did Indian Ocean LIPs related to the breakup of Gondwana tap a similar source and show a similar temporal and geochemical evolution to coeval and older Pacific LIPs?
Key questions include:
- What are the age, origin, and temporal and geochemical evolution of the uppermost AP volcanic sequences?
- How homogeneous/heterogeneous are the plateau lavas compared to other LIPS?
- Does the AP show geochemical affinities and melt production rates similar to other LIPS, e.g. Shatsky, Ontong Java, Manihiki, Hikurangi and Kerguelen Plateaus, requiring elevated mantle temperatures and/or lower mantle source (high 3He/4He and solar Ne isotope ratios) and thus the presence of a mantle plume from the lower mantle?Is this really a LIP or does a veneer of volcanic rocks overlie thinned continental crust? If the lavas passed through continental crust, they are likely to show geochemical signatures of continental crustal contamination.
- Do the uppermost AP lavas show evidence for subaerial volcanism?
- Is there evidence for late-stage volcanism on the AP as has been observed at Shatsky Rise and at the Ontong Java, Manihiki and Hikurangi Plateaus? How much younger is it compared to the main plateau stage and does it show compositional differences in major (alkalic vs. tholeiitic) and trace element and isotopic (e.g. more enriched) composition?
- How does Agulhas volcanism correlate to the separation of Africa and Antarctica and the formation of other plateaus in the South African gateway (e.g., the Mozambique Ridge)?
(B) Did sedimentation start immediately after crust emplacement at 100 Ma under subaerial conditions?
The following questions will be addressed:
- Which environment is indicated by the sediments deposited shortly after formation of the Agulhas Plateau – subaerial, shallow or deep water, warm or colder water?
- How did emplacement of the plateau influence the climatic, oceanic environments, and Earth’s biota? What can we deduce about water mass changes during early stages of the plateau’s evolution? Did emplacement of the Agulhas Plateau LIP interrupt an already incipient circulation?
(C) Did deep and intermediate water mass flow as well as climatic events leave their imprint in form of seismic reflections and unconformities?
(D) What was the palaeotemperature history at high southern latitudes across the rise and decline of Cretaceous Supergreenhouse and through the Paleocene?
Specific questions to address include:
- What was the thermal history of the Cretaceous and how was this linked to changes in atmospheric CO2, tectonic evolution, or some other forcing?Were surface water δ18O- and TEX86-derived palaeotemperatures at Agulhas Plateau cool enough at any time during the Cretaceous and Paleocene to allow for the growth of ice sheets in Antarctica? Can earth system models simulate the conditions recorded at Agulhas Plateau? What assumptions about CO2 levels are required to reproduce these conditions?
(E) Was the Cretaceous and Paleocene Southern Ocean area major source of deep water formation that strongly influenced climatic changes?
Specific questions to address include:
- What is the role and impact of the AP on the evolving communication of deep and intermediate water masses within and between ocean basins? Is there a co-development of Pacific-sourced deep-water circulation in the western (IODP 862-Pre) and eastern South Atlantic in the Paleocene?
- When and how did the opening of Southern Ocean gateways influence Late Cretaceous and Paleocene climate? Did early opening of the Drake Passage cause invigorated deep-water circulation and a strengthening of surface frontal systems in the eastern South Atlantic? Were late Paleocene temperatures in the high-latitude Atlantic cool enough to sustain ephemeral ice sheets on Antarctica.
(F) What forcing factors caused Cretaceous OAEs and what effects did these events have on the high latitude climate, oceanography and biota?
Key questions to address include:
- Was there an increase in Corg burial during the d13C excursion interval that defines Cretaceous OAEs?
- How did surface productivity, palaeotemperatures, the vertical thermal and d13C gradients, and bottom circulation change at AP across the OAE intervals?
- Did changes in AP microfossil assemblages correspond with observed OAE geochemical and sedimentological shifts and, if so, can the cause for those changes be determined?