Evaporite basins, orbital tilt, and the geomagnetic axial-dipolar field

Evans, D.A.D., Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes. Nature 444, 51-55 (2006); doi: 10.1038/nature05203

Two fundamental uncertainties have lingered over the problem of Proterozoic low-latitude glaciation. First, if Earth had a large (>54°) orbital obliquity (or tilt) at that time, the equator-to-pole temperature gradient would have been reversed, making tropical areas most susceptible to glaciation. Tropical glaciation would not indicate an extreme climate. Second, if the geomagnetic field had large non-dipolar (e.g., quadrupole, octupole) components, glacial deposits would have actually formed at higher paleolatitudes than inferred from paleomagnetic inclination data under the conventional assumption of a pure dipole field. One way to test both possibilities is through paleomagnetically-determined paleolatitudes of large-scale evaporite basins (e.g., basins containing deposits of salt, gypsum, potash salts, or their pseudomorphosed equivalents), which form in the subtropical arid zones under the descending limbs of the Hadley cells. (Exactly 50 years ago, the distribution of Phanerozoic evaporites was successfully used as an early test of continental drift. See E. Irving, Palaeomagnetic and palaeoclimatological aspects of polar wandering. Geofisica Pura e Applicada 33 (1956) 23-41.) In a recent article in the journal Nature, David A. D. Evans of Yale University has compiled the record of paleomagnetically-constrained evaporite basins back to 2.25 Ga (early Paleoproterozoic), four times longer than any previous compilation.

Proterozoic evaporite basins have a statistically significant volume-weighted concentration in the paleo-subtropics, slightly (~6°) equatorward of the mean paleolatitude for the last 250 Ma. This falsifies the predicted equatorial disposition of evaporite basins if the Proterozoic Earth had had a large orbital obliquity. (Obliquity cannot switch back and forth between large and small angles because the latter are stabilized by the Moon's gravitational attraction on the Earth's equatorial bulge.) Therefore, low-latitude glacial episodes cannot be attributed to a reversed climatic gradient. The slightly-lower mean paleolatitude of Proterozoic evaporite basins, compared with the Mesozoic-Cenozoic, could reflect a broadening of the Hadley cells with time, due to slowing of the Earth's rotation rate. Alternatively, it could mean there were minor non-dipolar components (e.g., <15% octupole) in the Proterozoic geomagnetic field. Somewhat larger components (<20% octupole) are possible in the interval between 550 and 370 Ma (late Ediacaran through late Devonian), when the mean paleolatitude of evaporite basins was ~9° lower than in the Mesozoic-Cenozoic. The non-dipole components would have an equatorward biasing effect on the paleolatitudes of Proterozoic glaciation, but not enough to substantially alter the climatological problem of low-latitude glaciation.

A large isotope anomaly before the Sturtian glaciation

McClay, G.A., Prave, A.R., Alsop, G.I., and Fallick, A.E., 2006. Glacial trinity: Neoproterozoic Earth history within the British-Irish Caledonides. Geology 34, 909-912; doi: 10.1130/G22694A.1

A third glacigenic formation and associated post-glacial cap dolostone have been found in the Dalradian Supergroup of the Irish-Scottish Caledonides (McClay et al., 2006). The Stralinchy-Reelan formations, composed of diamictite and ice-rafted debris, lie stratigraphically between the previously-known Port Askaig Tillite and the Inishowen-Loch na Cille ice-rafted debris beds. Carbon isotopes in the cap dolostone (basal Cranford Limestone) support a terminal Cryogenian (635 Ma) age for the glaciation, consistent with existing strontium isotope data indicating a "Sturtian" age for the older Port Askaig glaciation.

Directly below the Port Askaig Tillite is the Islay Limestone, which hosts a remarkable isotope anomaly, first recognized by Brasier & Shields (2000). Carbonate δ¹³C falls precipitously by ~10‰ to mantle-like values, and then rapidly recovers. Similar anomalies are found beneath "Sturtian" glacial deposits in the northern Canadian Cordillera (Coates Lake Group) and East Svalbard (Russøya Member), and also beneath "Marinoan" glacial deposits in many regions (the Trezona anomaly of Halverson et al., 2005). The occurrence of ~10‰ negative excusions in δ¹³C before both the "Sturtian" and "Marinoan" glaciations (purported snowball events) makes their close association unlikely to be a matter of chance.

Extreme winds and waves in glacial aftermath (January, 2005)

Allen, P.A. and Hoffman, P.F., 2005. Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature 433, 123-127.
Jerolmack, D.J. and Mohrig, D., 2005. Formation of Precambrian sediment ripples: Arising from P.A. Allen & P.F. Hoffman Nature 433, 123-127 (2005). Nature, 10.1038/nature04025.
Allen, P.A. and Hoffman, P.F., 2005. Formation of Precambrian sediment ripples: Reply to Jerolmack, D.J. & Mohrig, D. Nature, 10.1038/nature04025.

Sedimentologists from ETH-Zürich and Harvard University interpret “tepee” structures in younger Cryogenian cap dolostones as giant wave ripples, formed by extreme winds and waves in a pan-glacial aftermath. Hindcasting from gravitational wave theory, they suggest that the bedforms were produced at water depths of 200-400 meters by waves with maximum wave periods (21-30 seconds) significantly longer than those prevailing in today’s oceans. The reconstructed wave conditions could only have been generated by sustained wind speeds >20 meters per second (~3 times faster than present trade-wind speeds) in fetch-unlimited ocean basins. In a discussion published on-line, Jerolmack & Mohrig (MIT) suggest that the ripples were formed by hurricanes, not by prevailing winds. In response, the original authors note that multiple generations of ripples have similar azimuthal orientations, whereas hurricanes make landfall at different places and produce variable wind and wave orientations at any given location.

Iridium spikes suggest long-lived glaciations

Bodiselitsch, B., Koeberl, C., Master, S., and Reimold, W.U., 2005. Estimating duration and intensity of Neoproterozoic snowball glaciations from Ir anomalies. Science 308. 239-242.

High concentrations (~1 part per billion) of the platinum-group-element iridium (Ir) were discovered at the base of both the older and younger Cryogenian cap carbonates in four drill-cores from the Katangan succession of Zambia and southern Congo. The discoverers, from the University of Vienna (Austria) and Witwatersrand (South Africa), interpret the Ir as extraterrestrial (most terrestrial Ir is segregated in the iron core, leaving the silicate Earth depleted in Ir). Based on the average Cenozoic extraterrestrial Ir flux, they estimate that the Ir accumulated in glacial ice for ~12 million years and was then released when the glaciers melted. Using the Ordovician flux (considered anomalous), the estimated time-scale for ice entrainment is ~4 million years; using the Holocene flux (from Greenland ice), the time-scale is ~24 million years. If the discovery is replicated elsewhere and the Ir is truly extraterrestrial (testable with complete platinum-group-element ratios, osmium isotopes and spinel compositions), it will strongly support the snowball earth hypothesis.

Precise age for snowball termination (April, 2005)

Condon, D., Zhu, M., Bowring, S.A., Wang, W., Yang, A., and Jin, Y., 2005. U-Pb ages from the Neoproterozoic Doushantuo Formation, China. Science 308, 95-98.
Hoffmann, K.-H., Condon, D.J., Bowring, S.A., and Crowley, J.L., 2004. U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: constraints on Marinoan glaciation. Geology 32, 817-820, doi:10.1130/G20519.l

A team led by Dan Condon from MIT (Massachusetts Institute of Technology) succeeded in precisely dating the termination of the the younger Cryogenian glaciation. They determined (using isotope-dilution thermal-ionization mass spectrometry, or IDTIMS) that zircons from a volcanic ash layer within in the cap dolostone on the Nantuo glacials in South China have a concordia age of 635.2 ± 0.6 Ma. They previously obtained a statistically-indistinguishable age of 635.5 ± 0.5 Ma for correlative glacials in Namibia using the same method. The glacials and cap dolostones in both areas were correlated with the Marinoan succession in South Australia, where the base of the formal Ediacaran Period is defined at the base of the cap dolostone. If the correlations are correct, and assuming the deglaciation was rapid at low paleolatitudes (where all three areas existed), the termination of the Marinoan snowball earth and the start of the Ediacaran Period occurred at 635 Ma.

Microbial origin for vertical tubular structures in cap dolostones

Corsetti, F.A. and Grotzinger, J.P., 2005. Origin and significance of tube structures in Neoproterozoic post-glacial cap carbonates: example from Noonday Dolomite, Death Valley, United States. Palaios 20, 348-363.

Tubular structures filled by sediment and/or cement occur profusely in younger Cryogenian (Marinoan) cap dolostones in different areas (California, NW Canada, western Brazil and throughout Namibia). They were previously interpreted as gas or fluid escape pathways on account of their consistent paleovertical orientation, irrespective of the tilt of host layering. They occur exclusively within layered, non-skeletal, organo-sedimentary buildups known as stromatolites. Frank Corsetti (University of Southern California) and John Grotzinger (California Institute of Technology) reexamined the classic examples in the Noonday cap dolostone of the Death Valley area, originally studied by Preston Cloud and Lauren Wright. Corsetti and Grotzinger's painstaking analysis lends support for a microbial-ecological explanation for the tubular structures, which are strikingly similar to those in correlative cap dolostones on other paleocontinents.

Big spiny explosion in Australia

Grey, K., 2005. Ediacaran palynology of Australia. Association of Australian Paleontologists, Memoir 31, Canberra, 439 p.

Kath Grey’s long-awaited monograph on the Ediacaran palynomorphs (organic-walled microfossils separated after masceration) of Australia has arrived. Based on extensive deep drill coring throughout central Australia, she documents the explosive increase in the size, diversity, ornamentation and turnover rate among eukaryotic marine plankton near the stratigraphic horizon (lower Bunyeroo Formation and equivalents) marked by ejecta from the Acraman impact crater on the Gawler craton of South Australia. A broadly contemporaneous (early Ediacaran) explosion of acanthomorphic (spiny) palynomorphs is observed in Europe and South China, and has been interpreted as a response to the evolution of microscopic multicellular animals capable of eukaryote predation (Peterson & Butterfield, PNAS, 102, 9547-9552, 2005).

Improved carbon isotope record for Neoproterozoic seawater (Sept/Oct 2005)

Halverson, G.P., Hoffman, P.F., Schrag, D.P., Maloof, A.C., and Rice, A.H.N., 2005. Toward a Neoproterozoic composite carbon-isotope record. Geological Society of America Bulletin 117, 1181-1207, 10.1130/B25630.1

Galen Halverson and colleagues compiled high-resolution carbon isotope records from carbonate-dominated successions in Australia, Canada, Svalbard and Namibia into a composite δ¹³C record for Neoproterozoic seawater. The most striking feature of the new record is that for 170 My prior to the younger Cryogenian (Marinoan) glaciation, seawater was enriched in ¹³C on average by ~5‰ compared with Proterozoic and Phanerozoic norms. This could indicate that the organic carbon burial flux, as a fraction of the total (organic plus carbonate) carbon burial flux, was twice the normal value, or possibly that a huge reservoir of dissolved organic carbon existed in the ocean at that time (Rothman et al., 2003, PNAS 100, 8124-8129). Lynchpins of the new compilation are negative isotopic excursions ~800 Ma (Bitter Springs stage) and ~650 Ma, preceding the Marinoan glaciation (Trezona anomaly).

CO2-rich glacial aftermath inferred from pH-proxy record

Kasemann, S.A., Hawkesworth, C.J., Prave, A.R., Fallick, A.E., and Pearson, P.N., 2005. Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change. Earth and Planetary Science Letters 231, 73-86.

Geochemists and geologists based in the U.K. report that the boron isotope ratio δ¹¹B in a marine carbonate section in Namibia drops significantly across the younger Cryogenian (Marinoan) glaciation. The drop continues through the post-glacial cap dolostone. As continental weathering tends to lower the δ¹¹B of seawater and hydrothermal exchange tends to raise it, the observed change is unlikely to reflect a change in the composition of seawater. Rather, the authors suggest that the change in δ¹¹B was caused by a drop in pH associated with CO₂ build-up. Using calcium isotope data from the same section to estimate the relative change in the continental weathering flux, they suggest that CO₂ levels may have reached 0.1 bar (100,000 ppm, or ~333 times pre-industrial concentration) in the glacial aftermath.

Sea level and the BIF-glacial connection

Kump, L.R. and Seyfried, W.E., Jr., 2005. Hydrothermal Fe fluxes during the Precambrian: effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers. Earth and Planetary Science Letters 235, 654-662.

The association of banded iron formation (BIF) and glacial marine deposits was long-ago attributed by Henno Martin (1965) to “oxygen deficiency in stagnating bottom waters caused by an ice cover”. However, the steady-state iron-to-sulfur ratio (Fe:S) must exceed 0.5 or all iron will be removed as pyrite (FeS₂), not BIF (Fe₂O₃). Lee Kump (Pennsylvania State University) and Wilfred Seyfried (SUNY Buffalo) show that a large fall in sea level caused by a snowball earth would boost the Fe:S in hydrothermal vent fluids because of lower water pressure. The spotty distribution of glacial-hosted BIF may reflect regional variation in Fe:S in a poorly-mixed, ice-covered ocean.

Shining a light on snowball earth

Olcott, A.N., Sessions, A.L., Corsetti, F.A., Kaufman, A.J., and de Oliveira, T.F., 2005. Biomarker evidence for photosynthesis during Neoproterozoic glaciation. Science 310, 471-474.

A team led by Alison Olcott (University of Southern California) discovered biomarkers (molecular fossils) diagnostic of prokaryotic photosynthesis in drill-cores of organic-rich black shale from within the older Cryogenian (Sturtian) glacial succession of the Sao Francisco platform (Bambuí Basin) of southeastern Brazil. The existence of organic matter in the original sediment is strengthened by the presence of glendonites, calcite pseudomorphs after the morphologically distinctive mineral ikaite (CaCO₃.6H₂O), which forms in organic-rich sediments only, where diagenetic organic remineralization produces alkalinity and orthophosphate, the latter known to inhibit calcite formation in favor of ikaite. So does the existence of photosynthetic biomarkers in strata sandwiched between glacial marine diamictites repudiate tropical sea-glaciers in favor of tropical sea ice that is thin or absent? This depends on which part of the glacial cycle the organic-rich strata represent. If the glacial maximum, the question can be answered in the affirmative. But glacial deposits on Quaternary marine shelves and platforms commonly date from the glacial retreat, because lowered sea-level and expanded ice-sheets made them areas of erosion during glacial maxima. On the West Antarctic Peninsula, ice-shelf collapse appears to trigger glacier surge. In these scenarios, it would be a mistake to take evidence for photosynthesis in the glacial deposits as a constraint on the presence or thickness of marine ice during the glacial maximum.

Astronomical origin for snowball earths?

Pavlov, A.A., Toon, O.B., Pavlov, A.K., Bally, J., and Pollard, D., 2005. Passing through a giant molecular cloud: “Snowball” glaciations produced by interstellar dust. Geophysical Research Letters 32, L03705, 10.1029/2004GL021890

Alexander Pavlov (University of Colorado) and others proposed an astronomical theory for snowball earth episodes. They note that the solar system encounters a dense (>2000 H atoms cm^-3) giant molecular cloud (GMC) every 109 years on average (and GMC’s of average density every 108 years), with greatest probability every ~140 My when the solar system crosses the galactic spiral arms where GMC’s are concentrated. They estimate that tropospheric dust loading during dense GMC encounters would lower radiative forcing by >8 W/m2 for ~200,000 years. This is close to the forcing required to convert the present climate to a snowball earth. The change in forcing upon entering a GMC is rapid enough that it cannot be compensated by silicate-weathering feedback. The authors conclude that in its 4.5 Gyr of existence, the solar system encountered ~4 high-density GMC’s, which could have triggered snowball earths, and ~15 lower-density (~1000 H atoms cm^-3) GMC’s capable of causing moderate ice ages. They propose that GMC-triggered snowball earths might leave an isotopic signal, elevated ²³⁵U/²³⁸U ratio, at the base of post-glacial cap carbonates.

First big-game hunters

Peterson, K.J. and Butterfield, N.J., 2005. Origin of the Eumetazoa: testing ecological predictions of molecular clocks against the Proterozoic fossil record. Proceedings of the U.S. National Academy of Sciences 102, 9547-9552.

Kevin Peterson (Dartmouth College) and Nick Butterfield (Cambridge University) postulate that a sudden increase in size, diversity, ornamentation and turnover among organic-walled microplankton, which occurred early in the Ediacaran Period, was a response to the evolution of eumetazoa, the first predators of eukaryotes. The timing is broadly consistent with the age of the last common ancestor of eumetazoa and calcisponges, as estimated from extant descendents using an invertebrate-calibrated molecular “clock”. As there is no early Ediacaran (635-600 Ma) fossil record of eumetazoa, it must be assumed that they were themselves microscopic and free-floating or free-swimming.

Thin equatorial sea-ice solution for snowball earth with dynamic sea glaciers

Pollard, D. and Kasting, J.F., 2005. Snowball Earth: a thin-ice solution with flowing glaciers. Journal of Geophysical Research 110, C07010, 10.1029/2004JC002525

Dave Pollard and Jim Kasting (Pennsylvania State University) simulated a snowball earth in which thin (<2 m) equatorial sea-ice can coexist with sea-glacier dynamics, provided that the exposed equatorial ice is free of bubbles (i.e., mature marine ice). If bubble-rich ice (i.e., consolidated snow) is substituted, the equatorial ice thickens (>100 m) as in previous models. In the new simulation, the equatorial zone of thin ice is ~2200 km wide and separated from km-thick sea-glaciers by a narrow transition zone lying directly equatorward of the snow line. “Healthy” rates of photosynthesis could occur beneath the thin ice and the threshold CO₂ radiative forcing required for glacial termination is only 8% as high as that originally calculated for a snowball earth. The authors include an excellent review of the status of sea-glacier dynamics in snowball earth simulations.