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In the early 20th century, after contributing major advances in calculating radiation forcing on planetary bodies, Milutin Milankovitch the Serbian mathematician took up the challenge of explaining why Earth has experienced recurrent episodes of glaciation. Influenced by the ideas of his predecessors, Milankovitch developed a theory that centered on the notion that summertime temperature at high northern latitudes is the most important influence on the advance and retreat of glaciations. The calculations revealed a periodicity in summer insolation that had a reasonable correspondence with what was then known about the occurrence of ice ages. From that was born the elemental foundation of the orbital theory of the ice ages. That theory evolved over the next three decades while retaining the fundamental tenant that summer season insolation at the higher northern latitudes determines Earth’s climate variability. Scientists of the day were skeptical, and it was not until the 1960s that new techniques became available to test the temporal predictions of Milankovitch’s theory. The orbital theory gained support in the 1950s and 60s when methods for paleoclimate reconstructions documented an orbital-like recurrence pattern of cold and warm climate conditions spanning the past 2.5 million years. Accompanying the documentation of Earth’s climate rhythmicity from marine archives have been advances in other areas, including ice core records of atmospheric CO2 that pose challenges to the original orbital theory, namely what role have variations in atmospheric CO2 played in dictating the transitions from warm to cold and, what caused orbital scale variations in greenhouse gas concentrations. In this contribution we review the current state of knowledge about the Earth’s carbon cycle on glacial/interglacial timescales and explore how new information has begun to shed light on the long-standing goal to understand Earth’s natural climate rhythmicity. The findings presented here highlight the need to expand research on Earth’s geologic processes that influence the carbon budget on glacial timescales. And with this comes a new hypothesis that incorporates geologic processes in orbital scale climate cycles. 

The prevailing hypothesis to explain pCO2 rise at the last glacial termination calls upon enhanced ventilation of excess respired carbon that accumulated in the deep sea during the glacial. Recent studies argue lower [O2] in the glacial ocean is indicative of increased carbon respiration. The magnitude of [O2] depletion was 100–140 μ mol/kg at the glacial maximum. Because respiration is coupled to δ13C of dissolved inorganic carbon (DIC), [O2] depletion of 100–140 μ mol/kg from carbon respiration would lower deep water δ13CDIC by ∼1‰ relative to surface water. Prolonged sequestration of respired carbon would also lower the amount of 14C in the deep sea. We show that Pacific Deep Water δ13CDIC did not decrease relative to the surface ocean and Δ14C was only ∼50‰ lower during the late glacial. Model simulations of the hypothesized ventilation change during deglaciation lead to large increases in δ13CDIC, Δ14C, and ε14C that are not recorded in observations. 

ABSTRACT There is a growing database of radiocarbon ( 14 C) reconstructions from biogenic carbonate taken from marine sediment cores being used to investigate changing ocean circulation and carbon cycling at the end of the last great ice age. Reported here are 14 C results from a marine core taken in the Makassar Straits of the western equatorial Pacific that was intended to test whether there was evidence of geologic carbon release to the ocean during the glacial termination. A thorough investigation of planktic and benthic 14 C ages with stable isotopes and CT-scans revealed extensive burrowing in the upper 2 m of the core that displaced younger sediments downward by more than half a meter into the glacial section of the core. The vertical displacement is evident in both planktic and benthic fossils. However, the extent of displacement and the stratigraphic disturbance became evident only after multiple measurements of different species and genera. A CT-scan prior to sampling would be an effective screening tool to avoid sampling problem cores such as this. 

In paleoceanography, carbon and oxygen stable isotope ratios from benthic foraminifera are used as tracers of physical and biogeochemical properties of the deep ocean. We present the first version of the Ocean Carbon Cycling working group database,  of stable isotope ratios of oxygen and carbon from benthic foraminifera from deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-20 ky before present (BP)) to the Holocene (<10 ky BP) with a particular focus on the early last deglaciation (20-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models. Sites with at least millennial resolution were preferred, because the main goal is to resolve ocean changes associated with the last deglaciation on at least millennial timescales. Software tools were produced to access and analyze the data, and are included with this publication. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data in the compilation, even though its coverage is still sparse in many ocean regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation.</p> Custom python scripts to read and analyze the data base may be found in https://github.com/juanmuglia/OC3-python-scripts and in OC3-python-scripts.zip in this repository. plots_d13c.pdf and plots_d18o.pdf contain time series for all sites and available age models. 

Abstract We present the first version of the Ocean Circulation and Carbon Cycling (OC3) working group database, of oxygen and carbon stable isotope ratios from benthic foraminifera in deep ocean sediment cores from the Last Glacial Maximum (LGM, 23-19 ky) to the Holocene (<10 ky) with a particular focus on the early last deglaciation (19-15 ky BP). It includes 287 globally distributed coring sites, with metadata, isotopic and chronostratigraphic information, and age models. A quality check was performed for all data and age models, and sites with at least millennial resolution were preferred. Deep water mass structure as well as differences between the early deglaciation and LGM are captured by the data, even though its coverage is still sparse in many regions. We find high correlations among time series calculated with different age models at sites that allow such analysis. The database provides a useful dynamical approach to map physical and biogeochemical changes of the ocean throughout the last deglaciation. 

Abstract. During the early part of the last glacial termination (17.2–15 ka) and coincident with a ∼35 ppm rise in atmospheric CO2, a sharp 0.3‰–0.4‰ decline in atmospheric δ13CO2 occurred, potentially constraining the key processes that account for the early deglacial CO2 rise. A comparable δ13C decline has also been documented in numerous marine proxy records from surface and thermocline-dwelling planktic foraminifera. The δ13C decline recorded in planktic foraminifera has previously been attributed to the release of respired carbon from the deep ocean that was subsequently transported within the upper ocean to sites where the signal was recorded (and then ultimately transferred to the atmosphere). Benthic δ13C records from the global upper ocean, including a new record presented here from the tropical Pacific, also document this distinct early deglacial δ13C decline. Here we present modeling evidence to show that rather than respired carbon from the deep ocean propagating directly to the upper ocean prior to reaching the atmosphere, the carbon would have first upwelled to the surface in the Southern Ocean where it would have entered the atmosphere. In this way the transmission of isotopically light carbon to the global upper ocean was analogous to the ongoing ocean invasion of fossil fuel CO2. The model results suggest that thermocline waters throughout the ocean and 500–2000 m water depths were affected by this atmospheric bridge during the early deglaciation. 

Northern Iraq was the political and economic center of the Neo-Assyrian Empire (c. 912 to 609 BCE)—the largest and most powerful empire of its time. After more than two centuries of regional dominance, the Neo-Assyrian state plummeted from its zenith (c. 670 BCE) to complete political collapse (c. 615 to 609 BCE). Earlier explanations for the Assyrian collapse focused on the roles of internal politico-economic conflicts, territorial overextension, and military defeat. Here, we present a high-resolution and precisely dated speleothem record of climate change from the Kuna Ba cave in northern Iraq, which suggests that the empire’s rise occurred during a two-centuries-long interval of anomalously wet climate in the context of the past 4000 years, while megadroughts during the early-mid seventh century BCE, as severe as recent droughts in the region but lasting for decades, triggered a decline in Assyria’s agrarian productivity and thus contributed to its eventual political and economic collapse. 

Abstract The prevailing hypothesis to explain pCO₂rise at the last glacial termination calls upon enhanced ventilation of excess respired carbon that accumulated in the deep sea during the glacial. Recent studies argue lower [O₂] in the glacial ocean is indicative of increased carbon respiration. The magnitude of [O₂] depletion was 100–140 µ mol/kg at the glacial maximum. Because respiration is coupled toδ¹³C of dissolved inorganic carbon (DIC), [O₂] depletion of 100–140 µ mol/kg from carbon respiration would lower deep waterδ¹³C_DICby ∼1‰ relative to surface water. Prolonged sequestration of respired carbon would also lower the amount of¹⁴C in the deep sea. We show that Pacific Deep Waterδ¹³C_DICdid not decrease relative to the surface ocean and Δ¹⁴C was only ∼50‰ lower during the late glacial. Model simulations of the hypothesized ventilation change during deglaciation lead to large increases inδ¹³C_DIC, Δ¹⁴C, andε¹⁴C that are not recorded in observations. 

Abstract In the face of ongoing marine deoxygenation, understanding timescales and drivers of past oxygenation change is of critical importance. Marine sediment cores from tiered silled basins provide a natural laboratory to constrain timing and implications of oxygenation changes across multiple depths. Here, we reconstruct oxygenation and environmental change over time using benthic foraminiferal assemblages from sediment cores from three basins across the Southern California Borderlands: Tanner Basin (EW9504‐09PC, 1,194 m water depth), San Nicolas Basin (EW9504‐08PC, 1,442 m), and San Clemente Basin (EW9504‐05PC,1,818 m). We utilize indicator taxa, community ecology, and an oxygenation transfer function to reconstruct past oxygenation, and we directly compare reconstructed dissolved oxygen to modern measured dissolved oxygen. We generate new, higher resolution carbon and oxygen isotope records from planktic (Globigerina bulloides) and benthic foraminifera (Cibicides mckannai) from Tanner Basin. Geochemical and assemblage data indicate limited ecological and environmental change through time in each basin across the intervals studied. Early to mid‐Holocene (11.0–4.7 ka) oxygenation below 1,400 m (San Clemente and San Nicolas) was relatively stable and reduced relative to modern. San Nicolas Basin experienced a multi‐centennial oxygenation episode from 4.7 to 4.3 ka and oxygenation increased in Tanner Basin gradually from 1.7 to 0.8 ka. Yet across all three depths and time intervals studied, dissolved oxygen is consistently within a range of intermediate hypoxia (0.5–1.5 ml L⁻¹[O₂]). Variance in reconstructed dissolved oxygen was similar to decadal variance in modern dissolved oxygen and reduced relative to Holocene‐scale changes in shallower basins.