High-resolution global bathymetry grids for key Cretaceous and early Cenozoic climate stages

Brief description

Ocean currents are strongly controlled by seafloor topography. Recent studies have shown that small-scale features with slopes steeper than 0.05° significantly affect subsurface eddy velocities and the vertical structure of ocean circulation patterns. Such slope gradients represent the majority of the present-day oceanic basins. Modeling past oceanographic conditions for key climate stages requires similarly detailed paleo seafloor topography grids, in order to capture ocean currents accurately, especially for ocean models with sufficient resolution (<0.1°) to resolve eddies. However, existing paleobathymetry reconstructions use either a forward modeling approach, resulting in global grids lacking detailed seafloor roughness, or a backward modeling technique based on sediment backstripping, capturing realistic slope gradients, but for a spatially restricted area. Both approaches produce insufficient boundary conditions for high-resolution global paleo models. Here, we compute high-resolution global paleobathymetry grids, with detailed focus on the Southern Ocean, for key Cretaceous and early Cenozoic climate stages. We backstrip sediments from the modern global bathymetry, allowing the preservation of present-day seafloor slope gradients. Sediment isopach data are compiled from existing seismo-stratigraphic interpretations along the Southern Ocean margins, and expanded globally using total sediment thickness information and constant sedimentation rates. We also consider the effect of mantle flow on long-wavelength topography. The resulting grids contain realistic seafloor slope gradients and continental slopes across the continent-ocean transition zones that are similar to present-day observations. Using these detailed paleobathymetry grids for high-resolution global paleo models will help to accurately reconstruct oceanographic conditions of key climate stages and their interaction with the evolving seafloor.

Lineage

Maintenance and Update Frequency: quarterly

Statement: The paleobathymetry in this study is reconstructed for 38 Ma, using the plate tectonic model of Matthews et al. (2016)(31) in a paleomagnetic reference frame(32,33). Bathymetry at latitudes >40 °S is reconstructed following Hochmuth et al. (2019)(19), using sediment backstripping(34) with the software BALPAL(35). The grid is extended to the north (northern boundary at 25 °S and 0 °S, see Section 2.1) using the paleobathymetry of Baatsen et al. (2016)(36). The transition between both grids is smoothed to avoid artificial ‘jumps’ in the bathymetry. The maximum depth is set to 5500m. We use an approach that reconstructs ‘backwards’ in geological time, where sediment packages deposited since 38 Ma are removed from the present-day bathymetry(37), the plates reconstructed to their paleopositions(31), and sea level(38) and dynamic topography(39) changes are accounted for. Compared to ‘forward’ modelling techniques(40), this approach allows the preservation of realistic bathymetric features of seafloor roughness and small-scale, detailed geometry, such as fracture zones and seamounts, which are similar to the present-day, within the resulting paleogrid. Recent studies have shown that these small-scale features with slopes steeper than 0.05° significantly affect subsurface eddy velocities and the vertical structure of ocean circulation patterns(21,41). For the backstripping method, sediment thickness information is derived from seismo-stratigraphic interpretations, using seismic reflection and drilling data in the Southern Ocean(e.g., 42-46). Identified key seismic reflectors are converted from two-way travel time into depth below seafloor utilizing sonobuoy data and seismic reflection stacking velocities. Post-38 Ma sediments are ‘backstripped’ whilst underlying sedimentary material is decompacted. Sediment decompaction is calculated using the relationship between porosity and burial depth(47) for sand/silt in shelf and ooze in abyssal regions of the Southern Ocean. Isostatic rebound of the underlying crust resulting from the sediment removal is calculated after Airy’s law. Thermal subsidence through time is corrected, using the cooling model after Stein and Stein (1992)(48), for oceanic crust and large igneous provinces, and McKenzie (1978)(49), for extended continental crust. Changes in sea level(38) and dynamic topography(39) (Model 6) are included in our paleobathymetry calculation. Seafloor that has been subducted since the Eocene is incorporated into the grid by using data from global models (Nazca Plate(36)) and regional paleobathymetric reconstructions (Scotia Sea(50)). The paleodepths of both gateways (Tasmanian Gateway - TG, and Drake Passage - DP) are modified, whilst preserving the overall gateway geometries (Figure 1b, c). Both gateways’ deepest points are set to 300 m, 600 m, 1000m/1500m (for DP/TG, respectively) following previously published pre-EOT depth approximations with TG at 300 m(51), 600 m and 1500 m(52); and DP at 300 m(53), 600 m(54) and 1000 m(54).