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 ocean temperatures of up to 7–8 ∘C. The causes of this cooling are primarily thought to be due to tectonic plate movements driving changes in large-scale ocean circulation patterns, and hence heat redistribution, in conjunction with a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study, we assess the potential to constrain the evolving patterns of global ocean circulation and cooling over the last 15 Ma by assimilating a variety of marine sediment proxy data in an Earth system model. We do this by first compiling surface and benthic ocean temperature and benthic carbon-13 (δ13C) data in a series of seven time slices spaced at approximately 2.5 Myr intervals. We then pair this with a corresponding series of tectonic and climate boundary condition reconstructions in the cGENIE (“muffin” release) Earth system model, including alternative possibilities for an open vs. closed Central American Seaway (CAS) from 10 Ma onwards. In the cGENIE model, we explore uncertainty in greenhouse gas forcing and the magnitude of North Pacific to North Atlantic salinity flux adjustment required in the model to create an Atlantic Meridional Overturning Circulation (AMOC) of a specific strength, via a series of 12 (one for each tectonic reconstruction) 2D parameter ensembles. Each ensemble member is then tested against the observed global temperature and benthic δ13C patterns. 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 the cGENIE model's climate sensitivity and that it incorporates the effects of all greenhouse gases. We find that reproducing the observed long-term cooling trend requires a progressively declining greenhouse gas forcing in the model. In parallel to this, the strength of the AMOC increases with time despite a reduction in the salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and to lowering polar temperatures, both caused by CO2-driven global cooling. We also find that a closed CAS from 10 Ma to present shows better agreement between benthic δ13C patterns and our particular series of model configurations and data. A final outcome of our analysis is a pronounced ca. 1.5 ‰ decline occurring in atmospheric (and ca. 1 ‰ ocean surface) δ13C that could be used to inform future δ13C-based proxy reconstructions.

The Late Miocene Biogenic Bloom : A globally distributed but not an ubiquitous event

“…The link between the LMBB and LMCIS supposes that the input of nutrients from the continents would produce a peak in dissolution and therefore a decrease in δ 13 C values of dissolved inorganic carbon (Berger H, 1981;Diester-Haass et al, 2005;Bickert et al, 2004), yet this hypothesis is not consistent with our compilation, which shows no dissolution event. The LMCIS could also be a consequence of a change in global ocean circulation, in particular the contribution of NADW, which would result in a greater difference in δ 13 C between deep waters from the north and those from the south (Hodell & Venz-Curtis, 2006;Butzin et al, 2011;Thomas & Via, 2007;Poore et al, 2006;Crichton et al, 2021). Considering the timing of the LMCIS (7.5 to 6.7 Ma), this hypothesis would support the NADW intensification scenario for the origin of the LMBB.…”

Increased Biogenic Calcification and Burial Under Elevated pCO2 During the Miocene: A Model‐Data Comparison

“…Here we use the recent Miocene simulations developed by Crichton et al. (2021) as the basic configuration of our work. This particular implementation of the cGENIE model comprises mid‐Miocene continental and surface boundary conditions derived from fully coupled GCM experiments.…”

“…Salinity flux adjustment is then used to find the circulation pattern that best reproduces the Miocene carbon isotope records (please refer to Section 2.2 of Crichton et al. (2021) for details of model configurations). In this work, we keep all parameters the same as in the original study except for modifications on p CO 2 levels, global pelagic carbonate production rates, and weathering alkalinity flux.…”

“…In order to force changes in pelagic carbonate production, we manipulate PIC:POC ratio in each scenario so that organic productivity can remain the same as Crichton et al. (2021). The distribution of δ 13 C of benthic foraminifera is a function of the organic biological pump and ocean circulation.…”

System Architecture and Planetary Obliquity: Implications for Long-term Habitability

“…The simultaneous impact of the alternate eccentricity, obliquity, and climatic precession cycles on the surface temperature and sea ice extent are simulated with the GENIE Earth system model comprised of a coupled 2D Energy Moisture Balance Model (EMBM), 3D frictional geostrophic ocean model (Edwards & Marsh 2005;Marsh et al 2011), and a thermodynamic sea ice model (Weaver et al 2001). The model, coupled to models of atmospheric chemistry and marine biogeochemistry, has previously been used to simulate successfully recent glacial and interglacial conditions (Marsh et al 2006;Ma & Tian 2014;Kemppinen et al 2019), to reconstruct a wide variety of other paleoclimate states throughout Earth's geologic history (Donnadieu et al 2006;Meyer et al 2008;Panchuk et al 2008;Crichton et al 2021), for comparison between steady-state and transient runs (Lunt et al 2006), and to simulate changing surface conditions over a multi-million-year timescale in response to astronomical forcing (Vervoort et al 2021).…”