Linda A. Hinnov

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Changes in productivity and redox conditions in the Panthalassic Ocean during the latest Permian

The Gujo-Hachiman section in central Japan provides a rare window into environmental conditions within the Panthalassic Ocean, which encompassed more than half the Earth’s surface at 251 Ma. The section is characterized by a sharp transition from green-gray organicpoor cherts to black siliceous shales in the uppermost Permian. Normalization to the clay fraction demonstrates that apparent increases in the concentrations of organic matter and trace metals above this transition were due primarily to the loss of a diluent biogenic (radiolarian) silica fl ux and only secondarily to a small shift toward more reducing bottom waters. In the black shale, pyrite abundance increases by a factor of ~30× and is dominated by framboidal grains of probable syngenetic origin. These observations suggest that the expansion of lowoxygen conditions within the Panthalassic Ocean was focused within the oxygen-minimum zone rather than at the seafl oor. Such a pattern implies that (1) changes in nutrient fl uxes and primary productivity rates, rather than stagnation of oceanic circulation, were a key factor infl uencing oceanic redox conditions around the Permian-Triassic boundary, and (2) large regions of the Panthalassic Ocean underwent only limited redox changes, providing potential refugia for marine taxa that survived into the Triassic.

Cyclostratigraphy and astrochronology

Abstract: The Milankovitch theory that quasi-periodic oscillations in the Earth-Sun position have induced significant 10 4 -10 6 year variations in the Earth’s stratigraphic record of climate is widely acknowledged. This chapter summarizes the Earth’s astronomical parameters, the nature of astronomically forced solar radiation, fossil astronomical signals in the stratigraphic record, and the use of these signals in calibrating geologic time.

Middle Triassic orbital signature recorded in the shallow-marine Latemar carbonate buildup (Dolomites, Italy)

A new time-frequency analysis of sea-level–controlled carbonate-platform cycles in the Middle Triassic Latemar massif (Dolomites, Italy) reveals a strong depositional signature with characteristics of dominant forcing by climatic precession. Modes corresponding to long and short precession components at 1/(21.7 k.y.) and 1/(17.6 k.y.) underwent amplitude modulations matching Earth9s orbital eccentricity with major frequency components at 1/(400 k.y.), 1/(125 k.y.), and 1/(98 k.y.). Obliquity appears as a minor component at 1/(35.4 k.y.). The Latemar signature thus constitutes the oldest pristine Milankovitch signature yet observed in the geologic record. Its fidelity rivals that of the Pliocene-Pleistocene record originally used to confirm the theory of orbitally forced climates. This evidence deepens a widely noted disagreement between radiometric and cyclostratigraphic time scales for the Latemar buildup. The Latemar cycles indicate that orbitally forced sea-level oscillations were operative in the ice-free Middle Triassic hothouse world.

Cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences

Over the past 25 yr, the science of stratigraphy has evolved to include time-correlative data from vastly disparate components of the Earth system. Not least of these is the global signal afforded by cyclostratigraphy, which has recorded the evolution of Earth’s astronomical (“Milankovitch”) forcing of insolation and the paleoclimate system. Fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies, geochemistry, mineralogy, rock magnetism, color, etc. In step with the documentation of astronomically forced paleoclimate from ever-older older geologic times, innovations in computational science have provided ever-longer high-accuracy astronomical model “targets” that can be used for time scale calibration. The Earth’s orbit is affected by motions of other planets, notably the orbital perihelia of Venus and Jupiter, which impose a dominant 405 k.y. eccentricity cycle on Earth’s orbital evolution. The large mass of Jupiter stabilizes this cycle over hundreds of millions of years. The cyclostratigraphic record of 405 k.y. cycles is therefore often used to correct chronologies affected by variable sedimentation. Earth’s shape and rotation rate are influenced by tidal dissipation and climate friction; these effects affect Earth’s precession rate through time. Thus, a record of Earth-Moon evolution is also embedded in cyclostratigraphy. The geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high-precision radioisotope dating, which today has the potential to define the ages of stratigraphic horizons with 2σ uncertainties at the scale of a precession cycle. Precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system. Cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca. 50 Ma limit of accurate astronomical solutions, and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion. Astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time. Finally, space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of Mars, with patterns suggestive of astronomical forcing analogous to Earth.

Astronomical tuning of the Aptian Stage from Italian reference sections

A high-resolution grayscale series of the pelagic Fucoid Marls (Piobbico core, central Italy) shows strong, pervasive lithological rhythms throughout the Aptian interval. A hierarchy of centimeter- to meter-scale cycles characterizes the rhythms; when calibrating ~1 m cycles to Earth’s 405 k.y. orbital eccentricity cycle, these rhythms correspond to the periods of the eccentricity, obliquity, and precession index. Tuning to orbital eccentricity cycles provides a high-resolution time scale for the Aptian. Correlation to the Cismon core (northern Italy) extends the tuning to the Aptian-Barremian boundary. The tuning indicates a minimum duration of 13.42 m.y. for the Aptian Stage, where previous estimates range from 6.4 to 13.8 m.y. The combined Aptian–Albian astronomical tuning of the entire 77-m-long Piobbico core (and part of the Cismon core) provides a 25.85-m.y.-long astronomically calibrated time scale for Earth history.

Interhemispheric space-time attributes of the Dansgaard-Oeschger oscillations between 100 and 0 ka

The persistence, stability and interhemispheric phasing of the Dansgaard–Oeschger (DO) climate oscillation over the last glacial period (0–100 ka) has been evaluated in oxygen isotope records of three polar ice cores (GRIP, GISP2, and Byrd Station) and a midlatitude deep-sea core from the North Atlantic Ocean (MD95-2042). The results show that DO oscillations in atmospheric conditions occurred in both northern and southern polar ice records, although in the southern records the oscillations had at most only ca. one-tenth the power of those in the north. The DO oscillations first appeared duringMarine Isotope Stage (MIS) 4, and the average spectral power of the northern hemisphere DO oscillations increased markedly during MIS 3 (30–38 ka). The DO mode in the GISP2 record is confined to the frequency band 1/(1.59 kyr) to 1/(1.37 kyr), but in the GRIP record, the mode exhibits strong frequency splittingover a band that is wider by ca. 50%. Time-frequency analysis shows that in GRIP the DO mode underg oes a frequency modulation that is phase-locked with the Earth’s obliquity cycle; this modulation does not appear in nearby GISP2. In the North Atlantic marine record, DO oscillations behaved somewhat differently, appearingsporadically duringMIS 5 and 4. The planktonic DO oscillations increased in spectral power duringMIS 3, leadingpeak power in the GISP2 record by ca. 3 kyr. DO oscillations were relatively stable in all five records duringMIS 3; they could not be detected unequivocally in any of the records duringthe Holocene (0–11 ka). At other times, the DO mode in all of the records was amplitude-modulated by Earth’s orbital parameters. Finally, interhemispheric phasingof DO oscillations over 10–90 ka was assessed between the Byrd Station and GISP2 records, and between the benthic and planktonic records of Core MD95-2042. Coherency studies reveal an apparent time lead of Byrd DO oscillations over GISP2 by 384770 yr (2s level), and of the North Atlantic benthic over planktonic DO oscillations by 208733 yr. This apparent time lead of the southern over the northern temperature proxies is likely the consequence of the distinctive harmonic shapes affecting the northern (rectangular) vs. southern (triangular) DO oscillations; the actual northern–southern relationship, as suggested by modelling and other recent data studies, is most probably in simple antiphase (cross phase of 1801). r 2002 Published by Elsevier Science Ltd.

Strategies for assessing Early{Middle (Pliensbachian{Aalenian) Jurassic cyclochronologies

There are a number of fundamental problems in assessing the astronomically forced cyclostratigraphy of the Jurassic Period. First, Jurassic geochronology is not well constrained, due to a general scarcity of radiometric dates, inferior precision of the existing ones, and large inaccuracies in stratigraphic constraints. These problems are particularly troublesome in the Early to Middle Jurassic cyclic carbonates of the Colle di Sogno section in the Lombardy Pre–Alps, Italy. Second, Jurassic sedimentary formations have undergone significant diagenesis, and again, the Colle di Sogno carbonates, with their location on the southern edge of the Alpine fold belt, are no exception. This prevents the recovery of the primary geochemical proxies needed to evaluate the relationship between cyclic sedimentation and orbital forcing. Finally, even when a strong case can be made for orbitally forced cycles, a direct calibration between the Jurassic and existing theoretical orbital solutions is not yet possible. However, at Colle di Sogno new nannofossil biostratigraphy provides the opportunity to make significant headway in recognizing Jurassic signals consistent with orbital theory, and to propose cyclochronologic estimates for a portion of the Pliensbachian, and for the combined Toarcian–Aalenian Stages. We evaluate the presence of orbital signals in the sequence through the use of frequency modulation analysis of the cycles, as well as by spectral analysis of lithologic rank time–series reconstructions. The results show that the Pliensbachian Domaro Limestone contains a dual–component precession–like signal that undergoes frequency variation indicative of modulation by orbital eccentricity. The signal subsequently evolves into a single–component signal with frequency perturbations similar to those of the obliquity variation in the overlying Toarcian–Aalenian Sogno Formation. This switch in response to the fundamental orbital modes occurs during the deposition of the Lower Toarcian black shale, a time of significant global environmental change. The biostratigraphy at Colle di Sogno has well–defined Pliensbachian–Toarcian and Toarcian–Aalenian boundaries, allowing a cyclochronologic minimum estimate of 11.37 ±0.05 (1σ) Ma for the duration of the Toarcian–Aalenian interval (not accounting for stratigraphic and taxonomic noise). This compares well with the 13.1 ± 2.8 (1σ) Ma estimated for this same stratigraphic interval using the current radiometric geochronology. The underlying precession–forced Pliensbachian Domaro Limestone limestone–shale couplets suggest a time of formation for the sequence of least 5 Ma, with an age of onset in the jamesoni or polymorphus subzone.

Time-calibrated Milankovitch cycles for the late Permian

An important innovation in the geosciences is the astronomical time scale. The astronomical time scale is based on the Milankovitch-forced stratigraphy that has been calibrated to astronomical models of paleoclimate forcing; it is defined for much of Cenozoic–Mesozoic. For the Palaeozoic era, however, astronomical forcing has not been widely explored because of lack of high-precision geochronology or astronomical modelling. Here we report Milankovitch cycles from late Permian (Lopingian) strata at Meishan and Shangsi, South China, time calibrated by recent high-precision U–Pb dating. The evidence extends empirical knowledge of Earth’s astronomical parameters before 250 million years ago. Observed obliquity and precession terms support a 22-h length-of-day. The reconstructed astronomical time scale indicates a 7.793-million year duration for the Lopingian epoch, when strong 405-kyr cycles constrain astronomical modelling. This is the first significant advance in defining the Palaeozoic astronomical time scale, anchored to absolute time, bridging the Palaeozoic–Mesozoic transition.

Stratigraphic continuity and fragmentary sedimentation: the success of cyclostratigraphy as part of integrated stratigraphy

Abstract The Milankovitch theory of climate change is widely accepted, but the registration of the climate changes in the stratigraphic record and their use in building high-resolution astronomically tuned timescales has been disputed due to the complex and fragmentary nature of the stratigraphic record. However, results of time series analysis and consistency with independent magnetobiostratigraphic and/or radio-isotopic age models show that Milankovitch cycles are recorded not only in deep marine and lacustrine successions, but also in ice cores and speleothems, and in eolian and fluvial successions. Integrated stratigraphic studies further provide evidence for continuous sedimentation at Milankovitch time scales (104 years up to 106 years). This combined approach also shows that strict application of statistical confidence limits in spectral analysis to verify astronomical forcing in climate proxy records is not fully justified and may lead to false negatives. This is in contrast to recent claims that failure to apply strict statistical standards can lead to false positives in the search for periodic signals. Finally, and contrary to the argument that changes in insolation are too small to effect significant climate change, seasonal insolation variations resulting from orbital extremes can be significant (20% and more) and, as shown by climate modelling, generate large climate changes that can be expected to leave a marked imprint in the stratigraphic record. The tuning of long and continuous cyclic successions now underlies the standard geological time scale for much of the Cenozoic and also for extended intervals of the Mesozoic. Such successions have to be taken into account to fully comprehend the (cyclic) nature of the stratigraphic record.