Cenozoic climate changes have been linked to tectonic activity and variations in atmospheric CO2 concentrations. Here, we present Miocene and Pliocene sensitivity experiments performed with the climate model COSMOS. The experiments contain changes with respect to paleogeography, ocean gateway configuration, and atmospheric CO2 concentrations, as well as a range of vertical mixing coefficients in the ocean. For the mid-Miocene, we show that the impact of ocean mixing on surface temperature is comparable to the effect of the possible range in reconstructed CO2 concentrations. In combination with stronger vertical mixing, relatively moderate CO2 concentrations of 450 ppmv enable global-mean surface, deep-water, and meridional temperature characteristics representative of mid-Miocene Climatic Optimum (MMCO) reconstructions. The Miocene climate shows a reduced meridional temperature gradient and reduced seasonality. In the case of enhanced mixing, surface and deep ocean temperatures show significant warming of up to 5–10°C and an Arctic temperature anomaly of >12°C. In the Pliocene simulations, the impact of vertical mixing and CO2 is less important for the deep ocean, which we interpret as a different sensitivity dependence on the background state and mixed layer dynamics. We find a significant reduction in surface albedo and effective emissivity for either a high level of atmospheric CO2 or increased vertical mixing. Our mixing sensitivity experiments provide a warm deep ocean via ocean heat uptake. We propose that the mixing hypothesis can be tested by reconstructions of the thermocline and seasonal paleoclimate data indicating a lower seasonality relative to today.

The Thirty Years' War (1618-1648) had significant consequences for both human populations and natural ecosystems in Europe. To gain a better understanding of its impact on the local aquatic environment, we analyzed a sediment core from Stadtsee, a lake located in the city of Bad Waldsee, Upper Swabia, using a variety of aquatic biotic and abiotic proxies, including bioindicators, biomarkers, and sedimentological data. Through this multi-proxy approach, we infer changes in aquatic communities and their environment during this tumultuous period. Our findings indicate that this war had a marked impact on the lake ecosystem, as evidenced by changes in diatom assemblages, with declining diatom concentrations and progressively decreasing contributions of hypereutrophic species. These changes suggest a lowering in nutrient supply, possibly due to a reduced load of human and animal waste. This is supported by decreasing productivity (low Si/Ti) and declining δ¹³C_org values. It appears that the war led to a lake recovery and an improvement in water quality, as suggested by the presence of three bryozoans Plumatella species and the absence of Cristatella mucedo, indicating reduced nutrient levels and a well-oxygenated environment, in agreement with high Mn/Fe ratios. Additionally, the presence of chironomids, such as Corynoneura arctica-type and C. intersectus-type, point to a concomitant decline in temperature. This is consistent with preliminary results obtained by the lipid paleothermometer HDI₂₆. Our study thus highlights the valuable insights provided by bioindicators, biomarkers and sediment geochemistry in a concerted approach about water quality and the health of the Stadtsee ecosystem during the Thirty Years' War.

Lake Petén Itzá, Guatemala, is one of the oldest lakes in northern Central America. Its sedimentary record contains the signal in precipitation variability of the last four interglacial-glacial cycles (420-14 cal ka BP). Changes in rainfall during the last 80 kyr have been widely described in the lake’s record and associated with shifts in the Intertropical Convergence Zone (ITCZ) and the Atlantic Meridional Oceanic Circulation (AMOC). However, there is limited knowledge regarding the hydrological responses of Petén Itzá prior to this time interval due to a poorly constrained chronology. Here, we provide a significantly improved chronology allowing a more detailed analysis of changes in runoff, evaporation, and vegetation throughout the last 420 kyr. Magnetic susceptibility, bulk density, elemental geochemistry and pollen data indicate that glacial periods were characterized by dry conditions, generally associated with low runoff, high evaporation and temperate environments. On the contrary, interglacial periods suggest wetter conditions with high runoff, low evaporation and dominance of tropical vegetation. Our results are consistent with other paleo-precipitation records in the region, such as the marine records ODP 1239 from the eastern equatorial Pacific and the ODP 1002 from Cariaco Basin, suggesting that the runoff variability at Petén Itzá may be associated with the long-term latitudinal migration of the ITCZ. Frequency analyses of our data show the presence of 100, 40 and 21 kyr periodicities and thus an orbital control. Our study serves as a first reference point in the transregional study of the late Quaternary climate in northern Central America.