Effect of Vegetation on the Late Miocene Ocean Circulation

Gerrit Lohmann,Torsten Bickert,Martin Butzin

doi:10.3390/jmse3041311

Abstract

We examine the role of the vegetation cover and the associated hydrological cycle on the deep ocean circulation during the Late Miocene (~10 million years ago). In our simulations, an open Central American gateway and exchange with fresh Pacific waters leads to a weak and shallow thermohaline circulation in the North Atlantic Ocean which is consistent with most other modeling studies for this time period. Here, we estimate the effect of a changed vegetation cover on the ocean general circulation using atmospheric circulation model simulations for the late Miocene climate with 353 ppmv CO2 level. The Late Miocene land surface cover reduces the albedo, the net evaporation in the North Atlantic catchment is affected and the North Atlantic water becomes more saline leading to a more vigorous North Atlantic Deep Water circulation. These effects reveal potentially important feedbacks between the ocean circulation, the hydrological cycle and the land surface cover for Cenozoic climate evolution.

Highlights

The Eocene-Oligocene and the Mid-Miocene climate transitions are two major cooling steps in the Cenozoic climate evolution (Zachos et al, 2001, [1]) from greenhouse to “icehouse” climate conditions.Ocean circulation changes and atmospheric pCO2 variations are often cited as potential catalysts of these cooling events (DeConto and Pollard, 2003, [2])
For the late Miocene, simulated tropical trees are spread into subtropical Africa (North and South) and parts of Australia, whereas temperate trees are extended over Asia relative to present conditions
Bradshaw et al, (2015, [67]) suggest that climate sensitivity to CO2 forcing is directly affected by the palaeogeographic configuration and that the inferred climate sensitivity is higher for the late Miocene than we might expect for future climate because of the differences in vegetation distribution in conjunction with differences in ocean circulation and sea ice

Summary

Introduction

The Eocene-Oligocene and the Mid-Miocene climate transitions are two major cooling steps in the Cenozoic climate evolution (Zachos et al, 2001, [1]) from greenhouse to “icehouse” climate conditions.Ocean circulation changes and atmospheric pCO2 variations are often cited as potential catalysts of these cooling events (DeConto and Pollard, 2003, [2]). Tectonic reorganizations of gateways may have altered the large-scale ocean circulation, which in turn may have resulted in ice growth and global cooling (Kennett, 1977, [3]; Zachos et al, 2001, [1]). Carbon-13 proxy evidence (e.g., Wright and Miller, 1996, [4]; Billups, 2002, [5]) indicates pronounced ocean circulation changes in conjunction with the timing of tectonic events at critical ocean pathways like the Drake Passage, the Tasmanian Seaway, the Indonesian. In the case of the Miocene, elevated global-mean surface temperatures and weaker equator-to-pole temperature gradients are proposed (Greenwood and Wing, 1995, [11]; Crowley and Zachos, 2000, [12]; Pound et al, 2012, [13]). While numerical simulations exhibit rising global-mean temperatures for increasing greenhouse gas concentrations, they do not capture the reconstructed reduction in the meridional temperature gradient (Barron, 1987, [14]; Huber and Sloan, 2001, [15]; Micheels et al, 2011, [16])

Methods

Results

Discussion

Conclusion