Maureen E. Raymo

Influence of late Cenozoic mountain building on ocean geochemical cycles

In a steady-state ocean, input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes. For the elements strontium, calcium, and carbon, rivers provide the primary input, whereas marine biogenic sedimentation dominates removal. Dissolved fluxes in rivers are related to rates of continental weathering, which in turn are strongly dependent on rates of uplift. The largest dissolved fluxes today arise in the Himalayan and Andean mountain ranges and the Tibetan Plateau. During the past 5 m.y., uplift rates in these areas have increased significantly; this suggests that weathering rates and river fluxes may have increased also. The oceanic records of carbonate sedimentation, level of the calcite compensation depth, and delta/sup 13/C and delta/sup 87/Sr in biogenic sediments are consistent with a global increase in river fluxes since the late Miocene. The cooling of global climate over the past few million years may be linked to a decrease in atmospheric CO/sub 2/ driven by enhanced continental weathering in these tectonically active regions.

Matuyama 41,000-year cycles: North Atlantic Ocean and northern hemisphere ice sheets

Abstract During the middle Pleistocene, a change occurred in the climatic response of northern hemisphere ice sheets and the high-latitude North Atlantic Ocean to orbitally controlled variations in insolation. The dominant periodicity during the late Pliocene and early Pleistocene (2.47 Myr B.P. to about 0.735 Myr B.P.) was 41,000 years; thereafter, the dominance shifted to a period near 100,000 years. Although orbital forcing is the primary cause of each of these rhythmic responses, it does not explain the mid-Pleistocene shift of power. Changes in the solid boundary conditions of the ocean-air-ice system, and particularly in mountain elevation, are implicated.

Sea-level rise due to polar ice-sheet mass loss during past warm periods

Warming climate, melting ice, rising seas We know that the sea level will rise as climate warms. Nevertheless, accurate projections of how much sea-level rise will occur are difficult to make based solely on modern observations. Determining how ice sheets and sea level have varied in past warm periods can help us better understand how sensitive ice sheets are to higher temperatures. Dutton et al. review recent interdisciplinary progress in understanding this issue, based on data from four different warm intervals over the past 3 million years. Their synthesis provides a clear picture of the progress we have made and the hurdles that still exist. Science, this issue 10.1126/science.aaa4019 Reconstructing past magnitudes, rates, and sources of sea-level rise can help project what our warmer future may hold. BACKGROUND Although thermal expansion of seawater and melting of mountain glaciers have dominated global mean sea level (GMSL) rise over the last century, mass loss from the Greenland and Antarctic ice sheets is expected to exceed other contributions to GMSL rise under future warming. To better constrain polar ice-sheet response to warmer temperatures, we draw on evidence from interglacial periods in the geologic record that experienced warmer polar temperatures and higher GMSLs than present. Coastal records of sea level from these previous warm periods demonstrate geographic variability because of the influence of several geophysical processes that operate across a range of magnitudes and time scales. Inferring GMSL and ice-volume changes from these reconstructions is nontrivial and generally requires the use of geophysical models. ADVANCES Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise. Advances in our understanding of polar ice-sheet response to warmer climates have been made through an increase in the number and geographic distribution of sea-level reconstructions, better ice-sheet constraints, and the recognition that several geophysical processes cause spatially complex patterns in sea level. In particular, accounting for glacial isostatic processes helps to decipher spatial variability in coastal sea-level records and has reconciled a number of site-specific sea-level reconstructions for warm periods that have occurred within the past several hundred thousand years. This enables us to infer that during recent interglacial periods, small increases in global mean temperature and just a few degrees of polar warming relative to the preindustrial period resulted in ≥6 m of GMSL rise. Mantle-driven dynamic topography introduces large uncertainties on longer time scales, affecting reconstructions for time periods such as the Pliocene (~3 million years ago), when atmospheric CO2 was ~400 parts per million (ppm), similar to that of the present. Both modeling and field evidence suggest that polar ice sheets were smaller during this time period, but because dynamic topography can cause tens of meters of vertical displacement at Earth’s surface on million-year time scales and uncertainty in model predictions of this signal are large, it is currently not possible to make a precise estimate of peak GMSL during the Pliocene. OUTLOOK Our present climate is warming to a level associated with significant polar ice-sheet loss in the past, but a number of challenges remain to further constrain ice-sheet sensitivity to climate change using paleo–sea level records. Improving our understanding of rates of GMSL rise due to polar ice-mass loss is perhaps the most societally relevant information the paleorecord can provide, yet robust estimates of rates of GMSL rise associated with polar ice-sheet retreat and/or collapse remain a weakness in existing sea-level reconstructions. Improving existing magnitudes, rates, and sources of GMSL rise will require a better (global) distribution of sea-level reconstructions with high temporal resolution and precise elevations and should include sites close to present and former ice sheets. Translating such sea-level data into a robust GMSL signal demands integration with geophysical models, which in turn can be tested through improved spatial and temporal sampling of coastal records. Further development is needed to refine estimates of past sea level from geochemical proxies. In particular, paired oxygen isotope and Mg/Ca data are currently unable to provide confident, quantitative estimates of peak sea level during these past warm periods. In some GMSL reconstructions, polar ice-sheet retreat is inferred from the total GMSL budget, but identifying the specific ice-sheet sources is currently hindered by limited field evidence at high latitudes. Given the paucity of such data, emerging geochemical and geophysical techniques show promise for identifying the sectors of the ice sheets that were most vulnerable to collapse in the past and perhaps will be again in the future. Peak global mean temperature, atmospheric CO2, maximum global mean sea level (GMSL), and source(s) of meltwater. Light blue shading indicates uncertainty of GMSL maximum. Red pie charts over Greenland and Antarctica denote fraction (not location) of ice retreat. Interdisciplinary studies of geologic archives have ushered in a new era of deciphering magnitudes, rates, and sources of sea-level rise from polar ice-sheet loss during past warm periods. Accounting for glacial isostatic processes helps to reconcile spatial variability in peak sea level during marine isotope stages 5e and 11, when the global mean reached 6 to 9 meters and 6 to 13 meters higher than present, respectively. Dynamic topography introduces large uncertainties on longer time scales, precluding robust sea-level estimates for intervals such as the Pliocene. Present climate is warming to a level associated with significant polar ice-sheet loss in the past. Here, we outline advances and challenges involved in constraining ice-sheet sensitivity to climate change with use of paleo–sea level records.

Collapse of polar ice sheets during the stage 11 interglacial

Contentious observations of Pleistocene shoreline features on the tectonically stable islands of Bermuda and the Bahamas have suggested that sea level about 400,000 years ago was more than 20 metres higher than it is today. Geochronologic and geomorphic evidence indicates that these features formed during interglacial marine isotope stage (MIS) 11, an unusually long interval of warmth during the ice age. Previous work has advanced two divergent hypotheses for these shoreline features: first, significant melting of the East Antarctic Ice Sheet, in addition to the collapse of the West Antarctic Ice Sheet and the Greenland Ice Sheet; or second, emplacement by a mega-tsunami during MIS 11 (ref. 4, 5). Here we show that the elevations of these features are corrected downwards by ∼10 metres when we account for post-glacial crustal subsidence of these sites over the course of the anomalously long interglacial. On the basis of this correction, we estimate that eustatic sea level rose to ∼6–13u2009m above the present-day value in the second half of MIS 11. This suggests that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm period while changes in the volume of the East Antarctic Ice Sheet were relatively minor, thereby resolving the long-standing controversy over the stability of the East Antarctic Ice Sheet during MIS 11.

The impact of dynamic topography change on Antarctic ice sheet stability during the mid-Pliocene warm period

The evolution of the Antarctic ice sheet during the mid-Pliocene warm period (MPWP) remains uncertain and has important implications for our understanding of ice sheet response to modern global warming. The extent to which marine-based sectors of the East Antarctic Ice Sheet (EAIS) retreated during the MPWP is particularly contentious, with geological observations and geochemical analyses being cited to argue for either a relatively minor or a significant ice sheet retreat in response to mid-Pliocene warming. The stability of marine-based ice sheets is intimately linked to bedrock elevation at their grounding lines, and previous ice sheet modeling assumed that Antarctic bedrock elevation during the MPWP was the same as today with the exception of a correction for the crustal response to ice loading. However, various processes may have perturbed bedrock elevation over the past 3 m.y., most notably vertical deflections of the crust driven by mantle convective flow, or dynamic topography. Here we present simulations of mantle convective flow that are consistent with a wide range of present-day observables and use them to predict changes in dynamic topography and reconstruct bedrock elevations during the MPWP. We incorporate these elevations into a simulation of the Antarctic ice sheet during the MPWP and find that the correction for dynamic topography change has a significant effect on the stability of the EAIS within the marine-based Wilkes Basin, with the ice margin in that sector retreating considerably further inland (200–560 km) relative to simulations that do not include this correction for bedrock elevation.

Paleo Constraints on Future Sea-Level Rise

Sea-level rise predicted for the twenty-first century and beyond will become increasingly hazardous to coastal populations, economies, static infrastructure, and ecosystems. Accurately predicting the magnitude and rate of future sea-level rise at local, regional, and global scales is necessary to effectively plan for and manage this growing hazard. Sea-level reconstructions show how high and how fast sea level rose when Earth’s climate regime was similar to that anticipated in the immediate future. We draw upon examples from the past three million years, including the Pliocene (∼3 million years ago), the Last Interglacial period (Marine Isotope Stage 5e, ∼125,000xa0years ago), and the Common Era (last ∼2000xa0years) to provide a synopsis of what is known about sea-level rise during these past warm periods and highlight some of the benefits and challenges of using paleo sea-level data to predict future changes.

A model for ‘sustainable’ US beef production

Food production dominates land, water and fertilizer use and is a greenhouse gas source. In the United States, beef production is the main agricultural resource user overall, as well as per kcal or g of protein. Here, we offer a possible, non-unique, definition of ‘sustainable’ beef as that subsisting exclusively on grass and by-products, and quantify its expected US production as a function of pastureland use. Assuming today’s pastureland characteristics, all of the pastureland that US beef currently use can sustainably deliver ≈45% of current production. Rewilding this pastureland’s less productive half (≈135 million ha) can still deliver ≈43% of current beef production. In all considered scenarios, the ≈32 million ha of high-quality cropland that beef currently use are reallocated for plant-based food production. These plant items deliver 2- to 20-fold more calories and protein than the replaced beef and increase the delivery of protective nutrients, but deliver no B12. Increased deployment of rapid rotational grazing or grassland multi-purposing may increase beef production capacity.The US beef industry is regarded as environmentally unsustainable. Modelling a system where cattle subsist solely on grass and food industry by-products, the authors estimate that 45% of current production could be achieved without the use of any high quality cropland.

Giant boulders and Last Interglacial storm intensity in the North Atlantic

Significance The Last Interglacial was the last period of the Earth’s history when climate was warmer than preindustrial, with higher polar temperatures and higher sea levels. Based on geologic evidence in Bermuda and the Bahamas, studies suggest that during this period the North Atlantic was characterized by “superstorms” more intense than any observed historically. Here we present data and models showing that, under conditions of higher sea level, historically observed hurricanes can explain geologic features previously interpreted as evidence for more intense Last Interglacial storm activity. Our results suggest that, even without an increase in the intensity of extreme storms, cliffs and coastal barriers will be subject to significantly higher wave-induced energies under even modestly higher sea levels. As global climate warms and sea level rises, coastal areas will be subject to more frequent extreme flooding and hurricanes. Geologic evidence for extreme coastal storms during past warm periods has the potential to provide fundamental insights into their future intensity. Recent studies argue that during the Last Interglacial (MIS 5e, ∼128–116 ka) tropical and extratropical North Atlantic cyclones may have been more intense than at present, and may have produced waves larger than those observed historically. Such strong swells are inferred to have created a number of geologic features that can be observed today along the coastlines of Bermuda and the Bahamas. In this paper, we investigate the most iconic among these features: massive boulders atop a cliff in North Eleuthera, Bahamas. We combine geologic field surveys, wave models, and boulder transport equations to test the hypothesis that such boulders must have been emplaced by storms of greater-than-historical intensity. By contrast, our results suggest that with the higher relative sea level (RSL) estimated for the Bahamas during MIS 5e, boulders of this size could have been transported by waves generated by storms of historical intensity. Thus, while the megaboulders of Eleuthera cannot be used as geologic proof for past “superstorms,” they do show that with rising sea levels, cliffs and coastal barriers will be subject to significantly greater erosional energy, even without changes in storm intensity.

An early Pleistocene Mg/Ca‐δ18O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41‐kyr world

PUBLICATIONS Paleoceanography RESEARCH ARTICLE 10.1002/2016PA002956 Key Points: • Six ice sheet meltwater events identified in Gulf of Mexico seawater δ O record from 2.55-1.70 Ma • Events are typically long, occur late in benthic δ O deglaciations, and line up with summer insolation • This challenges view of early Pleistocene marine δ O as simply recording obliquity-driven Northern Hemisphere ice volume Supporting Information: • Supporting Information S1 • Supporting Information S2 Correspondence to: J. D. Shakun, jeremy.shakun@bc.edu Citation: Shakun, J. D., M. E. Raymo, and D. W. Lea (2016), An early Pleistocene Mg/Ca-δ O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41-kyr world, Paleoceanography, 31, 1011–1027, doi:10.1002/2016PA002956. Received 25 MAR 2016 Accepted 5 JUL 2016 Accepted article online 11 JUL 2016 Published online 25 JUL 2016 An early Pleistocene Mg/Ca-δ 18 O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41-kyr world Jeremy D. Shakun 1 , Maureen E. Raymo 2 , and David W. Lea 3 Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA, USA, 2 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA, 3 Department of Earth Science, University of California, Santa Barbara, CA, USA Early Pleistocene glacial cycles in marine δ 18 O exhibit strong obliquity pacing, but there is a perplexing lack of precession variability despite its important influence on summer insolation intensity – the presumed forcing of ice sheet growth and decay according to the Milankovitch hypothesis. This puzzle has been explained in two ways: Northern Hemisphere ice sheets instead respond to insolation integrated over the summer, which is mostly controlled by obliquity, or anti-phased precession-driven variability in ice volume between the hemispheres cancels out in global δ 18 O, leaving the in-phase obliquity signal to dominate. We evaluated these ideas by reconstructing Laurentide Ice Sheet (LIS) meltwater discharge to the Gulf of Mexico from 2.55-1.70 Ma using foraminiferal Mg/Ca and δ 18 O. Our δ 18 O sw record displays six prominent anomalies, which likely reflect meltwater pulses, and they have several remarkable characteristics: (1) their presence suggests that the LIS expanded into the mid-latitudes numerous times; (2) they tend to occur or extend into interglacials in benthic δ 18 O; (3) they generally correlate with summer insolation intensity better than integrated insolation forcing; and (4) they are perhaps smaller in amplitude but longer in duration than their late Pleistocene counterparts, suggesting comparable total meltwater fluxes. Overall, these observations suggest that the LIS was large, sensitive to precession, and decoupled from marine δ 18 O numerous times during the early Pleistocene – observations difficult to reconcile with a straightforward interpretation of the early Pleistocene marine δ 18 O record as a proxy for Northern Hemisphere ice sheet size driven by obliquity forcing at high latitudes. Abstract 1. Introduction The Milankovitch hypothesis holds that ice sheets are sensitive to the intensity of summer insolation, which depends on both the tilt of the earth – which varies with the 41-kyr obliquity cycle – and the seasonal distance to the sun – which varies with the 23-kyr precession cycle. The Milankovitch model has had consid- erable success in explaining late Pleistocene ice volume variations of the past one million years, cycles that are concentrated at eccentricity, precession and obliquity frequencies as well as their multiples [Huybers, 2011; Imbrie and Imbrie, 1980; Raymo, 1997]. Nonetheless, the marine δ 18 O record suggests that the immedi- ately preceding glacial cycles of the late Pliocene-early Pleistocene (3–1 Ma) occurred at the almost purely 41-kyr pacing (Figure 1b) [Huybers, 2007; Pisias and Moore, 1981; Raymo and Nisancioglu, 2003; Ruddiman et al., 1989]. This 41-kyr world is difficult to reconcile with the Milankovitch hypothesis – why is precession variabil- ity absent in the early Pleistocene if summer insolation intensity controls ice sheet mass balance? ©2016. American Geophysical Union. All Rights Reserved. SHAKUN ET AL. Two hypotheses have been suggested to rectify the apparent conflict between the ice volume changes pre- dicted by Milankovitch forcing and those actually observed in the marine δ 18 O record. The Integrated Insolation hypothesis points out that the most intense summers are also the shortest, since the Earth orbits faster when closer to the sun [Huybers, 2006]. Since these competing precession-driven effects, intensity ver- sus duration, nearly cancel out when integrated over the course of the summer, one might not expect to see a strong precession signal in ice volume variability. The Antiphase hypothesis instead argues that ice sheets are driven by both obliquity and precession (as expressed in summer insolation intensity), but while obliquity is in phase between the hemispheres (i.e., increased axial tilt causes stronger summers in both hemispheres), precession forcing is anti-phased (i.e., when one hemisphere’s summer occurs closest to the sun, the other’s summer occurs farthest from the sun six months later) [Raymo et al., 2006]. Therefore, if a record of global ice volume, such as marine δ 18 O or sea level, was recording ice volume changes in both hemispheres, it would EARLY PLEISTOCENE MELTWATER EVENTS

Reply to Hearty and Tormey: Use the scientific method to test geologic hypotheses, because rocks do not whisper

Hearty and Tormey (1) challenge our conclusions (2), incorrectly arguing that the megaboulders we discuss were shown to originate from the cliff bottom. A number of mischaracterizations are made by Hearty and Tormey (1) in their letter. First, we do not use a “tsunami wave model.” Second, we do not address the two other Bahamian landforms Hearty and Tormey (1) mention: their “superstorm” genesis interpretation [for which alternative hypotheses have been proposed (3, 4)] has no bearing on our (2) conclusions.nnHearty and Tormey’s (1) claim that the boulders have “fingerprints” based on “several physical criteria” and “data from multiple disciplines” is false. Only two mega-boulder “physical properties” were reported by Hearty … nn[↵][1]1To whom correspondence should be addressed. Email: arovere{at}marum.de.nn [1]: #xref-corresp-1-1