Traces of past oceanic and environmental conditions are stored in deep sea sediments as ‘proxy records’. When these ‘proxy records’ are deciphered, they can contain valuable information that can be used to reconstruct past chemical and physical conditions of the ocean throughout time. Though, these records can be distorted by a variety of chemical, physical, and biological processes including bioturbation, dissolution, and variations in sedimentation rate, complicating the relationship between stratigraphic depth and age of a geochemical proxy, and making it more difficult to translate past climates and environments. As CO2 levels continue to rise in our atmosphere, an understanding of how past extreme warming events, such as the Paleocene-Eocene Thermal Maximum (PETM, ~55.9 Ma), impacted environmental conditions and ecosystem dynamics can better inform how our modern ocean ecosystems will react to anthropogenic warming. During the PETM, a rapid injection of isotopically light carbon into the ocean and/or atmosphere, indicated by a negative carbon isotope excursion (CIE), resulted in global warming, changes in ocean ecosystem dynamics including a severe benthic foraminifera extinction, and global dissolution of deep-sea carbonates due to shoaling of the calcite compensation depth (CCD). This subsequent dissolution erased the onset of the PETM CIE at many deep sea locations, while the impact on the deep sea macrobenthic community remains unexplored. In this dissertation I investigate the response of the deep sea benthic and infaunal macrofauna response to the PETM by quantifying bioturbation intensity utilizing the ichnofabric index (ii) by identifying and measuring trace fossil community change in 46 deep sea sediment cores. I find global decreases in trace fossil community size, with over half of the locations ceasing bioturbation following the onset of the PETM. I utilize a novel burrow-specific sampling technique to investigate the movement of bulk sediment within discrete trace fossils. I find the most significant impact on the geochemical record occurs in concurrence with the CIE. I utilize neodymium isotopes in the North Atlantic at IODP site U1403 to investigate circulation patterns and find no change from Southern dominated deep water formation throughout the PETM.

There is ample evidence from benthic foraminiferal oxygen isotope records that the Earth’s climate experienced dramatic long-term variations from the late Paleocene to early-middle Eocene. Beginning from ~57 Ma and culminating in the Early Eocene Climate Optimum (EECO) between ~53-51 Ma, the recorded 1‰ decrease in δ18O indicates warming of ~4 to 5 °C, followed by the onset of long-term cooling at the early-middle Eocene boundary (~48 Ma). Benthic foraminifera also record major changes in carbon isotopes during this time period, with a decline in δ13C of ~2 to 2.5‰, indicating that the temperature changes coincided with profound changes in the carbon cycle. These trends are consistent with an overall increase in atmospheric CO2 for the duration of the event, perhaps caused by a protracted increase in volcanic outgassing or decrease in net organic carbon burial. However, there is a temporal offset between the benthic foraminiferal δ13C and δ18O signals, with the long-term minimum in δ13C significantly preceding peak global temperatures indicated by the long-term minimum in δ18O. If a global signal, this offset may suggest changes in the source of carbon fluxes across this interval of long-term global warming, rather than the existence of a single-source carbon forcing and resultant temperature response. To constrain possible causes of the offset, we first analyze the relative timing of isotope signals from each individual site within the Cramer et al. 2009 benthic foraminiferal stable isotope composite. Our analysis shows that trends in δ13C consistently lead trends in δ18O at each site. However, the length of the offset varies significantly between sites. We find that the duration of this offset is greatest at southern sites, potentially indicating that changing patterns of deep-water formation contribute to the apparent decoupling between δ13C and δ18O trends. As δ13C is a gauge of water mass age, spatial gradients in benthic foraminiferal δ13C can indicate regional differences in deep water formation, shedding light on the evolution of global circulation. We next take an additional investigative approach utilizing the cGENIE intermediate complexity Earth system model. We use cGENIE in order to test the impact of enhanced volcanic outgassing on spatial patterns in benthic δ13C and deep ocean temperature, and relate this to changes in the large-scale ocean overturning circulation.

Planetary climates are controlled by the delicate radiative balance between incoming and outgoing energy. Any perturbation to either side of the equation disrupts the surface temperature with implications for the physical environment and (bio)geochemical processes that in their turn impose additional alterations to the radiative energy balance. In this dissertation, I use numerical approaches to investigate the role of changing greenhouse gas concentrations and astronomical forcing (quasi-periodic variations in solar radiation via gradual changes in Earth's orbit and tilt) on the surface climate, carbon cycling, and the environmental consequences of associated feedback processes, on Earth and beyond.

First, I address the impact of carbon dioxide release to the atmosphere on geological timescales. Past carbon release events and episodes of global warming that are preserved in paleoclimate records are compared to an ensemble of Earth system model simulations to estimate the carbon input and output fluxes during past events and determine how the Earth responded to such perturbations. Next, I provide the reader with a thorough analysis of the impact of astronomical forcing and its imprint on the marine environment as simulated in an Earth system model. The transient modeling approach allows direct comparison between time-varying model output and multi-million-year-long paleoclimate records and thereby provides an opportunity to determine from model-data (mis)matches what feedback processes are the main drivers for variations in the climate-carbon system on astronomical timescales across tens of thousand to millions of years. Lastly, I examine with the use of orbital and obliquity simulations how the frequency and amplitude of astronomical cycles change as a function of planetary architecture and assess their impact on surface conditions and planetary habitability across million-year timescales. This research contributes to identifying exoplanets in other systems that may have been able to maintain habitable conditions over prolonged periods of time required for the development and evolution of life elsewhere in the universe.

The effects of changes to environments on Earth in the past have been well documented, and we have fossil evidence of how organisms responded to these changes. In the past, possible responses to environmental change have ranged from extinction and origination to migration to physical morphological changes. As we are currently facing the prospect of dramatic alterations to Earth’s environments, understanding how life responded in the past may be one of our best opportunities to gauge how future scenarios might play out.

Here I present new data detailing how two different groups of organisms were living in fluctuating environments at two times in Earth’s history. During the Ediacaran there was a unique subset of organisms with three-fold symmetry that went extinct before the beginning of the Cambrian. Although they were relatively abundant and diverse, representing ~12-15% of the taxonomic diversity of the second (White Sea) Ediacaran assemblage, no animals at any point afterwards have had tri-radially symmetric body plans. I show that most of these tri-radial taxa share many morphological and ecological similarities beyond their distinctive body plan and were likely related members of a tri-radial clade. Furthermore, there is evidence that multiple tri-radial taxa were generalists, able to survive in a wide variety of environments, indicating that a relatively sharp environmental change would have been necessary to eliminate this group.

An almost opposite scenario is true for deep-sea ostracodes during Eocene Thermal Maximum 2 (ETM2) in the equatorial and North Atlantic. Using high-resolution sampling from ocean drill cores (ODP Site 1258 and IODP Site U1409) I show that, although this was a relatively smaller event than the preceding Paleocene-Eocene Thermal Maximum, ostracodes respond in noticeable ways. Ostracode diversity and abundance (as measured by accumulation rate) remain low throughout the duration of ETM2. Additionally, the dominant species in these cores exhibit a size decrease that is also correlated with the event. Despite showing these clear signals of having been impacted by ETM2, no ostracode genera disappear completely from either of the sites, indicating that they were able to adapt and survive the environmental changes they faced.