Data-constrained assessment of ocean circulation changes since the middle Miocene in an Earth system model

Abstract

Abstract. Since the middle Miocene, 15 Ma (million years ago), the Earth’s climate has undergone a long-term cooling trend, characterised by a reduction in sea surface temperatures by over 6 °C, with 4 to 6 °C cooling occurring in the deep ocean. The causes of this cooling are primarily thought to be linked to changes in ocean circulation due to tectonic plate movements affecting ocean seaways, together with and a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study we assess the potential to constrain, using marine sediment proxy data, the evolving patterns of global ocean circulation and cooling of surface climate over the last 15 million years (Ma) in an Earth system model. We do this by compiling surface and benthic ocean temperature and benthic carbon-13 data in a series of seven time-slices spaced at approximately 2.5 million year intervals. We pair this with a corresponding series of seven tectonic and surface climate boundary condition reconstructions in the cGENIE (muffin release) Earth system model. In the cGENIE model, we adjust atmospheric CO2 together with the magnitude of North Pacific to North Atlantic salinity flux adjustment in a series of 2D parameter ensembles in order to match global temperature and benthic δ13C patterns in the model to the data. We identify that a relatively high CO2 equivalent forcing of 1120 ppm is required at 15 Ma in cGENIE to reproduce proxy temperature estimates in the model, noting that this CO2 forcing is dependent on cGENIEs climate sensitivity (which is as the present day) and that it incorporates the effects of all greenhouse gases. The required CO2 forcing progressively reduces throughout the subsequent six time slices delineating the observed long-term cooling trend. In order to match the evolving patterns of the proxy data, we require fundamental change in the mode of ocean circulation at 12.5 Ma with present-day-like benthic δ13C trends established by 10 Ma. We also find a general increasing strength of Atlantic overturning despite a reduction in salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and polar cooling caused by CO2-driven global cooling.

Decreasing Atmospheric CO2 During the Late Miocene Cooling

“…All of our sensitivity tests reproduce a similar decline in atmospheric CO 2 , confirming that the conclusion of a late Miocene CO 2 decline is robust to uncertainties in estimations of growth rate. Finally, our reconstructed values and trend fit a recent cGENIE model (Crichton et al, 2020), that predicts CO 2 levels of 400 ppm at 4.5 Ma and 800 ppm at 7.5 Ma (Figure 7).…”

“…Note the logarithmic scale of the y axis. Small box: long‐term trend in p CO 2 from the cGENIE modeling study by Crichton et al (2020) for a broader perspective. The two larger black stars are falling within the time period of this study.…”

“…from the Monte Carlo simulation, and it is disconnected to the youngest and oldest sample of the record because of the error bias discussed in section 5.4. Additional proxy data indicated as boron isotopes with red plus signs (Sosdian et al, 2018), dark green triangles (ODP 925) and light green dots (ODP 999) from recalculated ε p (Stoll et al, 2019), orange squares (ODP 846) from diatom ε p record (Mejía et al, 2017), black crosses from stomata index (Bai et al, 2015), and black stars from cGENIE modeling (Crichton et al, 2020). The red shaded bar indicates the abrupt end of the LMC and coincides with a sharp increase in atmospheric CO 2 .…”

Temperature controls carbon cycling and biological evolution in the ocean twilight zone

“…We implemented a series of configurations of the cGENIE Earth system model with appropriate paleogeography and optimized ocean circulation patterns for each target age (Table S1) (25,30,31). We further made use of a new version of the model biogeochemistry module that includes temperature-dependent nutrient uptake rates at the surface and respiration in the ocean interior (32).…”

Late Neogene evolution of modern deep-dwelling plankton

“…To facilitate a general discussion of near-surface changes, we divided the data latitudinally by calculating the arithmetic mean for low latitudes (<16° latitude), two mid-latitude bands (mid 1: 16° to 40°, mid 2: 40° to 56°) and high latitudes (>56°). The cGENIE simulations take account of changing boundary conditions including CO2 forcing, bathymetry and ocean circulation (Crichton et al, 2020). The model's ocean biological carbon pump is temperature dependent, where temperature affects both nutrient uptake rates at the surface and remineralization rates of sinking particulate organic matter down the water column (Crichton et al, 2021).…”