Early Pleistocene Obliquity‐Scale pCO2 Variability at ~1.5 Million Years Ago

In the early Pleistocene, global temperature cycles predominantly varied with ~41‐kyr (obliquity‐scale) periodicity. Atmospheric greenhouse gas concentrations likely played a role in these climate cycles; marine sediments provide an indirect geochemical means to estimate early Pleistocene CO2. Here we present a boron isotope‐based record of continuous high‐resolution surface ocean pH and inferred atmospheric CO2 changes. Our results show that, within a window of time in the early Pleistocene (1.38–1.54 Ma), pCO2 varied with obliquity, confirming that, analogous to late Pleistocene conditions, the carbon cycle and climate covaried at ~1.5 Ma. Pairing the reconstructed early Pleistocene pCO2 amplitude (92 ± 13 μatm) with a comparably smaller global surface temperature glacial/interglacial amplitude (3.0 ± 0.5 K) yields a surface temperature change to CO2 radiative forcing ratio of S[CO2]~0.75 (±0.5) °C−1·W−1·m−2, as compared to the late Pleistocene S[CO2] value of ~1.75 (±0.6) °C−1·W−1·m−2. This direct comparison of pCO2 and temperature implicitly incorporates the large ice sheet forcing as an internal feedback and is not directly applicable to future warming. We evaluate this result with a simple climate model and show that the presumably thinner, though extensive, northern hemisphere ice sheets would increase surface temperature sensitivity to radiative forcing. Thus, the mechanism to dampen actual temperature variability in the early Pleistocene more likely lies with Southern Ocean circulation dynamics or antiphase hemispheric forcing. We also compile this new carbon dioxide record with published Plio‐Pleistocene δ11B records using consistent boundary conditions and explore potential reasons for the discrepancy between Pliocene pCO2 based on different planktic foraminifera.Key PointsEarly Pleistocene pCO2 roughly varied with obliquity cyclesInterglacial pCO2 was similar in the early and late Pleistocene; glacial pCO2 declined over the mid‐Pleistocene transitionDiscrepancies between δ11B values and corresponding pCO2 estimates from G. ruber and T. sacculifer are observed and may indicate evolving vital effectsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147130/1/palo20675-sup-0004-2018PA003349-S03.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147130/2/palo20675.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147130/3/palo20675-sup-0002-2018PA003349-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147130/4/palo20675_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147130/5/palo20675-sup-0005-2018PA003349-S04.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147130/6/palo20675-sup-0003-2018PA003349-S02.pd

Marginal Reefs Under Stress: Physiological Limits Render Galápagos Corals Susceptible to Ocean Acidification and Thermal Stress

Ocean acidification (OA) and thermal stress may undermine corals' ability to calcify and support diverse reef communities, particularly in marginal environments. Coral calcification depends on aragonite supersaturation (Ω » 1) of the calcifying fluid (cf) from which the skeleton precipitates. Corals actively upregulate pHcf relative to seawater to buffer against changes in temperature and dissolved inorganic carbon, which together control Ωcf. Here we assess the buffering capacity of modern and fossil corals from the Galápagos Islands that have been exposed to sub-optimal conditions, extreme thermal stress, and OA. We demonstrate a significant decline in pHcf and Ωcf since the pre-industrial era, trends which are exacerbated during extreme warm years. These results suggest that there are likely physiological limits to corals' pH buffering capacity, and that these constraints render marginal reefs particularly susceptible to OA

Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions

This research is funded by NSF [OCE12-32987] to BH.The B/Ca ratio of planktic foraminiferal calcite, a proxy for the surface ocean carbonate system, displays large negative excursions during the Paleocene-Eocene Thermal Maximum (PETM, 55.9 Ma), consistent with rapid ocean acidification at that time. However, the B/Ca excursion measured at the PETM exceeds a magnitude that modern pH-calibrations can explain. Numerous other controls on the proxy have been suggested, including foraminiferal growth rate and the total concentration of Dissolved Inorganic Carbon (DIC). Here we present new calibrations for B/Ca vs. the combined effects of pH and DIC in the symbiont-bearing planktic foraminifer Orbulina universa, grown in culture solutions with simulated Paleocene seawater elemental composition (high [Ca], low [Mg], and low [B]T). We also investigate the isolated effects of low seawater total boron concentration ([B]T), high [Ca], reduced symbiont photosynthetic activity, and average shell growth rate on O. universa B/Ca in order to further understand the proxy systematics and to determine other possible influences on the PETM records. We find that average shell growth rate does not appear to determine B/Ca in high calcite saturation experiments. In addition, our “Paleocene” calibration shows higher sensitivity than the modern calibration at low [B(OH)4-]/DIC. Given a large DIC pulse at the PETM, this amplification of the B/Ca response can more fully explain the PETM B/Ca excursion. However, further calibrations with other foraminifer species are needed to determine the range of foraminifer species-specific proxy sensitivities under these conditions for quantitative reconstruction of large carbon cycle perturbations.Publisher PDFPeer reviewe

Pacific Ocean Pleistocene and Holocene surface temperature variability and implications for climate change

As humanity embarks on a global experiment in climate warming associated with increased levels of atmospheric greenhouse gases, we require a thorough understanding of the mechanisms, dynamics, and spatial extent of current and past environmental change on a range of timescales. In the past 1.5 million years (Pleistocene and Holocene), glacial-interglacial cycles emerge as the dominant pattern of orbital-scale global temperature change with millennial-scale oscillations superimposed. The ~100-ka glacial cycles are especially well recognized in the oceanographic and geologic records, though the climate-ocean dynamics and feedback mechanisms responsible for these glacial cycles are often a source of debate. Past sea-surface temperature (SST) is an especially useful indicator of past environmental conditions and a multitude of paleotemperature indicators exist. I make use of a number of geochemical signals for SST including the magnesium-calcium ratio of biogenic calcite, oxygen isotopes, and the alkenone unsaturation index from lipid ketones in order to better understand the mechanisms and dynamics of the current and past climate system.The California Current and the tropical Pacific are two of the major features that influence climate in California, the tropics, and across the globe through direct effects (e.g., upwelling, sea surface temperature changes) and atmospheric teleconnections. This dissertation examines past oceanic and atmospheric change in these two critical regions and linkages with high-latitude climate, insolation, and radiative greenhouse gas forcing. The coastal California chapter addresses questions regarding the regional and local strength of the California Current, upwelling intensity, and precipitation variability in the Holocene. Tropical Pacific chapters deal with the spatial distribution of tropical Pacific sea-surface temperatures as a reflection of oceanographic change and a driver of global climate. Each of these regions offers lessons for understanding climate variability from the late Pleistocene to the present and future climates

Multicentennial Agulhas leakage variability and links to North Atlantic climate during the past 80,000 years

New high-resolution sea surface temperature (SST) and sea surface salinity (SSS) estimates are presented from the Agulhas Bank slope in the Atlantic sector of the Agulhas Corridor using planktic foraminiferal (Globigerinoides ruber) δ18O and Mg/Ca-derived SST. By focusing on the last 80,000 years, this is the first fine-scale Agulhas leakage record that overlaps in time with much of the Greenland ice core record of abrupt climate changes in the North Atlantic region. The multicentennial profiles indicate instances of warm SST and/or increased SSS coincident with Northern Hemisphere cool periods, followed by Northern Hemisphere warming. These periods of enhanced SST and SSS in the Agulhas Corridor occur at the last glacial termination (T1) and during North Atlantic cold episodes associated with Heinrich (H) meltwater events. To a first-order approximation, the timing of maximal salinity events in relation to periods of North Atlantic freshwater perturbation is consistent with the concept suggested by climate models that enhanced Agulhas leakage provides for buoyancy compensation and can potentially trigger increased convective activity in the North Atlantic, thereby restoring Atlantic overturning circulation after relatively weak states

Dataset for paper "Southward shift and intensification of the intertropical convergence zone in the North Pacific across the mid-Pleistocene transition" submittted to Geophysical Research Letters

<p>Data of SST and SSS from <i>G. ruber</i> 18O and Mg/Ca of core ODP871 (5°33′N, 172°21′E, water depth 1255 m) in the central equatorial Pacific.</p&gt

Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions

The B/Ca ratio of planktic foraminiferal calcite, a proxy for the surface ocean carbonate system, displays large negative excursions during the Paleocene-Eocene Thermal Maximum (PETM, 55.9 Ma), consistent with rapid ocean acidification at that time. However, the B/Ca excursion measured at the PETM exceeds a magnitude that modern pH calibrations can explain. Numerous other controls on the proxy have been suggested, including foraminiferal growth rate and the total concentration of dissolved inorganic carbon (DIC). Here we present new calibrations for B/Ca versus the combined effects of pH and DIC in the symbiont-bearing planktic foraminifer Orbulina universa, grown in culture solutions with simulated Paleocene seawater elemental composition (high [Ca], low [Mg], and low total boron concentration ([B]T). We also investigate the isolated effects of low seawater [B]T, high [Ca], reduced symbiont photosynthetic activity, and average shell growth rate on O. universa B/Ca in order to further understand the proxy systematics and to determine other possible influences on the PETM records. We find that average shell growth rate does not appear to determine B/Ca in high calcite saturation experiments. In addition, our “Paleocene” calibration shows higher sensitivity than the modern calibration at low [B(OH)4−]/DIC. Given a large DIC pulse at the PETM, this amplification of the B/Ca response can more fully explain the PETM B/Ca excursion. However, further calibrations with other foraminifer species are needed to determine the range of foraminifer species-specific proxy sensitivities under these conditions for quantitative reconstruction of large carbon cycle perturbations

Palaeoclimate constraints on the impact of 2 °C anthropogenic warming and beyond

Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over centennial-to-millennial timescales that are often not covered by climate model simulations. Our observation-based synthesis of the understanding of past intervals with temperatures within the range of projected future warming suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2 °C. However, substantial regional environmental impacts can occur. A global average warming of 1-2 °C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of palaeo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.Financial support of the PAGES Warmer World Integrative Activity workshop by the Future Earth core project PAGES (Past Global Changes) and the Oeschger Centre for Climate Change Research, University of Bern, is gratefully acknowledged. Additional funding by PAGES was provided to the plioVAR, PALSEA 2, QUIGS, the 2k network, C-peat, Global Paleofire 2 and OC3 PAGES working groups contributing to the Integrated Activity (see http://www.pages.unibe.ch/science/intro for an overview of all former and active PAGES working groups)

Symbiont Photosynthesis and Its Effect on Boron Proxies in Planktic Foraminifera

Boron proxies in the calcium carbonate shells of planktic foraminifera are sensitive to seawater acidity, but B/Ca ratios and isotopic composition (i.e., δ11B) recorded by different foraminifer species grown under identical environmental conditions differ significantly and systematically. Specifically, Globigerinoides ruber displays higher B/Ca and δ11B than Trilobatus sacculifer and Orbulina universa. It has been hypothesized that these differences are caused by species‐specific rates of symbiont photosynthesis and habitat depth with greater symbiont photosynthesis elevating the microenvironmental pH of G. ruber relative to T. sacculifer and O. universa. Here we test this hypothesis by applying fast repetition rate fluorometry (FRRF), Chlorophyll a quantification, and symbiont counts in laboratory grown specimens of G. ruber (pink), T. sacculifer and O. universa to study species‐specific differences in symbiont photochemical quantum efficiencies. In addition, we report B/Ca shell profiles measured by laser ablation on the same specimens previously monitored by FRRF, and δ11B data of discrete populations of all three species grown under high and low light conditions in the laboratory. While the light experiments document that symbiont photosynthesis elevates pH and/or δ11B in the calcifying microenvironment of all three foraminifer species, the FRRF, Chl. a and symbiont abundance data are relatively uniform among the three species and do not scale consistently with intrashell B/Ca, or with observed species‐specific offsets in B/Ca or δ11B. Implications of these findings for foraminiferal physiology and biomineralization processes are discussed.Key PointsSymbiont photosynthesis raises pH in the microenvironment of planktic foraminiferaForaminifera species‐specific offsets in boron proxies are the same in laboratory culture and in the natural oceanic environmentSymbiont photosynthesis alone does not explain species‐specific boron proxy offsets in planktic foraminiferaPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/170833/1/palo21091.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/170833/2/palo21091_am.pd