I no longer consider myself a scientist. I am proud of my work, and created this page both as a repository of information and to provide links to collaborators who continue the research. This page has a popular science explanation of my work, as well as the published abstracts. Text last updated in 2007.
Snowball Earth: was our planet completely covered by glaciers?
I stand at the base of the Panamint Mountains in Death Valley, where the edge of a vanished ocean once lapped the nascent North American continent. Five thousand feet above my head is a series of boulder-choked canyons preserved in ancient silt beds, evidence of deep erosion. These canyons are strung like pearls across the California-Nevada border, crushed and sliced and torn apart by more than 500 million years of history.
I was first sent to the Panamints to trek up and down its canyons with a staff, measuring the thickness of the seemingly featureless green slate cliffs of the Johnnie Formation. Of indeterminable age - older than 750 million years, younger than 535 million years - the Johnnie Formation is part of the southwest's layer cake stratigraphy. My advisor hoped we could use intriguing thickness variations another student had found elsewhere in the Johnnie to help put the mountainous region around Death Valley back together. These ranges spread across the desert in the past 20 million years along a complex set of faults - perhaps matching up the canyons could help us puzzle the pieces back together.
But the canyons were so majestic in size and so varied in style that our original purpose developed a new focus. Filled with chaotic rock rubble some areas, pretty pink quartz pebbles in others, the biggest notch was almost 500 feet deep. What formed these canyons? A likely explanation is sea level changes caused by extreme glaciation. Extreme because this part of North America was apparently at sea level and relatively close to the equator around 600 million years ago. In present time, only very tall mountains have glaciers at these latitudes.
Our discovery was late to the party - by 1997, geologists had found signs of an ancient equatorial glaciation on every continent. Many lines of evidence lead them to believe these rocks were deposited within the same time frame near the equator. For further discussion of the Snowball Earth theory, see Web sites by the original theorists, Joe Kirschvink at Caltech and Paul Hoffman at Harvard, or visit Wikipedia. Try a Google search on the Johnnie Formation for the most recent research.
Neoproterozoic glacial record in the Death Valley region, California and Nevada.
Abolins-Mark; Oskin-Rebecca; Prave-Tony; Summa-Catherine; Corsetti-Frank-A
In: Great Basin and Sierra Nevada. GSA Field Guide. 2; Pages 319-335. 2000.
The Neoproterozoic succession in the Death Valley region contains a physical- and chemostratigraphic record of glaciation. Direct evidence for glaciation includes dropstones, glacially influenced diamictites, and cap carbonates in the Neoproterozoic Kingston Peak Formation. Within this formation, glacially influenced deposits and cap carbonates occur at two distinct horizons, suggesting at least two glacial episodes. Cap-like carbonates and sequence boundaries elsewhere in the succession may indicate additional glacial intervals. The basal Beck Spring Dolomite has facies and isotopic characteristics commonly associated with cap carbonates, and the rest of the succession is punctuated by numerous sequence boundaries including a prominent incised horizon in the uppermost Johnnie Formation. This horizon is locally overlain by carbonate with cap-like facies and isotopic characteristics. Together, these observations indicate at least two and possibly four distinct Neoproterozoic glaciations.
Three distinct Neoproterozoic glacial intervals recorded in western Laurentia and Australia.
Abolins-Mark-J; Charlton-Rebecca-L; Wernicke-Brian-P; Ripperdan-Robert-L
In: Geological Society of America, 1999 annual meeting. Abstracts with Programs - Geological Society of America. 31; 7, Pages 485. 1999.
New observations in the Johnnie Fm. in the Death Valley region, CA-NV, combined with those in other parts of the section, suggest at least three distinct Neoproterozoic glacial intervals. Throughout most of this region, the distinctive Rainstorm Member (pale red siltstone and carbonate with oolitic marker horizon) is the uppermost unit in the Johnnie Fm. Locally, the Rainstorm Member is reduced in thickness or absent, and a markedly different suite of lithologies occurs at the top, including granular sandstone, conglomerate, olive gray siltstone, and, locally, coarse diamictite. This suite has been assigned to the informal Conglomeratic Member (Charlton et al., 1997, GSA nat'l mtg abstr.). This member shares lithologies and lithofacies with incised valley fill within the Neoproterozoic Caddy Canyon Formation of Idaho and Utah.Before deposition of the Conglomeratic Member, incision locally removed some or all of the Rainstorm member, accounting for the abrupt thickness variations observed in the unit. Field observations suggest that incision involved a regional drop in relative sea level of at least 150 m followed by infilling with the Conglomeratic member. A drop in sea level of this magnitude is best explained by glacio-eustasy, since direct evidence for syn-depositional tectonism is absent. In addition, positive carbon isotope values in the Rainstorm Member are similar to those found in strata underlying Neoproterozoic glacial deposits.Taken together, the incised valleys in the uppermost Johnnie Formation and the two distinct diamictites in the underlying Kingston Peak Formation suggest three distinct glacial intervals. We speculate that the incised horizon in the uppermost Johnnie Fm. correlates with the incised horizon in the uppermost Caddy Canyon Formation of Idaho and Utah, and with diamictite in the Ice Brook Formation of the Mackenzie Mountains, Canada and the Yerelina Subgroup of southeastern Australia. According to this interpretation, Neoproterozoic sections in all of these regions record three distinct large-magnitude glaciations.
A major Neoproterozoic incision event near the base of the Cordilleran Miogeocline, southwestern Great Basin.
Charlton-Rebecca-L; Wernicke-Brian-P; Abolins-Mark-J
In: Geological Society of America, 1997 annual meeting. Abstracts with Programs - Geological Society of America. 29; 6, Pages 197. 1997.
In the southwestern Great Basin, a sequence boundary reflecting a major incision event within the upper part of the Neoproterozoic Johnnie Formation is recognized in the northern Panamint Range, the Resting Spring Range, and the Spring Mountains. Similar large-magnitude incision events have been recognized in latest Neoproterozoic successions in Utah (Caddy Canyon Quartzite), South Australia (Wonoka Fm.), and Namibia (Nama Gp.). These events indicate a rapidly changing environment just prior to the first appearance of multi-cellular life. The type 1 sequence boundary is defined at the base of a newly recognized succession mostly composed of monotonous olive-gray (5Y4/1) siltstone. The succession also includes a basal carbonate which is locally a coarse breccia (clasts up to 3 m) and near the top, interstratified siltstone, coarse sandstone and quartz-pebble conglomerate, in places containing carbonate boulders up to 13 m in maximum dimension. This sequence overlies the Rainstorm Member, previously regarded as the youngest part of the Johnnie. Summa (1993, M.I.T. Ph.D. thesis) first recognized incision of the Rainstorm Member (including truncation of the Johnnie oolite) in the southern Nopah Range. We have observed the basal carbonate of the sequence laterally truncating some 250 m of the Rainstorm over a distance of a few hundred meters in the west-central Resting Spring Range. Throughout the northern Panamint Range, the Rainstorm appears to be entirely missing, and the younger sequence is as much as 1000 m thick. Clasts of underlying units are common within the breccias and conglomerates, including the Rainstorm Member, the lower part of Johnnie, the Noonday Dolomite, and the Beck Spring Dolomite. If the upper Johnnie event is correlative with other major incision events it may provide an age constraint on the time of initiation of the Cordilleran miogeocline.
Active tectonics of the Laptev Sea, northeast Siberia, from ERS-1 derived gravity field.
In: AGU 1996 fall meeting. Eos, Transactions, American Geophysical Union. 77; 46, Suppl., Pages 703. 1996.