How often did it rain?

Geoscientists have become used to masses of climate data, often with better than 50 years resolution, from cores through ice sheets and sea-floor sediments.  But all of it is from some kind of proxy; oxygen isotopes for air temperature and land-ice volume, methane for humidity, dust for windiness, and so forth.  One aspect of both climate and the British obsession with weather is raininess, for which there is scant evidence.  How many rainy days occur in a British summer is interesting, but for studies of past climate evidence for the onset or disappearance of seasonality, and the annual intensity and duration of rainfall would be invaluable, if it could be had.  A piece of ingenious research shows that it is possible (Kano, A. et al. 2004.  High-resolution records of rainfall events from clay bands in tufa.  Geology, v. 32, p. 793-796).  Akihiro Kano and Japanese colleagues studied the well-known layering of tufa – carbonate veneers laid down in freshwater that has high dissolved bicarbonate and calcium ions.  In “hard-water” areas tufa can be deposited very quickly, at rates above a few millimetres per year, and it tends to be preserved, being quite tough.  So tufas have the potential for preserving annual records of various fluctuations.  Kano and colleagues saw that colour laminations represented clays deposited in the tufa when the water was turbid after prolonged rainfall.  To record the variations they simply measured fluorescent X-rays emitted by silicon when slices of tufa were examined in an electron microprobe – silicon is present in clays and silt, but not in carbonate minerals.  Because they used tufa deposited in recent times (1988-2002) they were able to correlate variations in clay content with detailed weather records from the site, thereby calibrating their method.  The match was very good and followed rainfall closely at the level of a few days.  Of 112 high rainfall days in the abnormally wet year of 1993, 100 showed up in the clay record.  So, tufas are potentially more revealing than even the annual growth rings in wood, and some tufa deposits preserve long records.

Details of the last interglacial climate

Worries about how anthropogenic warming will affect the course of the Holocene interglacial in which we live might be tempered or exacerbated by knowing what went on during the previous, Eemian interglacial that ended about 120 ka ago.  Data from cores through the Greenland and Antarctic ice sheets have been both ambiguous and plagued by resolution that does not show enough detail, but a core from a new position in Greenland seems to resolve both problems (North Greenland Ice Core Project members 2004.  High-resolution record of Northern Hemisphere climate extending into the last interglacial period.  Nature, v. 431, p. 147-151).  Uniquely, the NGRIP ice still preserves the annual snow layering as far back as 123 ka.  This is because the site shows little sign of the deformation at deep levels that characterised previous Greenland cores.  That is probably because the site lies above a zone of high heat flow through the underlying crust, so that the base of the ice has melted.  Melting helps prevent internal deformation, but that in itself is a surprise because the site was chosen because it is colder and drier at the surface than other sites.  The drilling objective was to penetrate older ice than the Eemian to give a fuller record than from earlier cores, yet anticipated poor time resolution.  The presence of resolvable annual records from depth was both a surprise and a bonus, although the melting had removed ice from the earliest part of the last interglacial.  Despite that, preliminary oxygen-isotope results from the NGRIP core suggest that the Eemian had a remarkably stable climate and one that was warmer than that of the Holocene by about 5ºC; maybe it is an analogue for climate evolution during a future, artificially warmed world.  That possibility stems from the observation that around 115 ka, North Atlantic climate suddenly warmed  Thereafter, interglacial conditions did not suddenly change to glacial, as happened several times during the course of the last glacial epoch, but took around five millennia after the sudden warming.  The authors make no claims that their preliminary data help resolve current fears of warming collapsing to glacial conditions in a matter of years to decades.  That grim scenario has been widely trumpeted both by the media and some climate scientists.  There is more to the Eemian than the period after 123 ka, and who knows what the eventual annual resolution will show up?  The data presented in the paper are from a coarse sampling of 55 cm that represents about 40 year intervals.

See also: Kuffey, K.M. 2004.  Into an ice age.  Nature, v. 431, p. 133-134

For and against “Snowball Earth”

Reputedly glaciogenic sediments in the Neoproterozoic are reckoned to represent at least three separate cold episodes, the Sturtian (~720 Ma), Marinoan (~600 Ma) and Varangerian (~580 Ma).  Sadly, the diamictites that characterise these episodes are not easily dated.  Only two have well-defined radiometric ages, the Gubrah Member in the Oman (713 Ma), said to be Sturtian, and the Gaskiers Formation of Newfoundland (580 Ma), a possible example of the Varangerian that is better exposed in northern Norway.  The truly whopping Sturtian and Marinoan diamictites of Australia are fitted to a global stratigraphy on the basis of carbon isotope variations, as are those of Namibia on which Paul Hoffman and colleagues stake their claims to “Snowball Earth” events. Another Hoffman, native to Namibia, and geochemists at MIT, have finally given a believable age to one of the Namibian diamictites (Hoffman, K.-H. et al 2004.  U-Pb zircon dates from the Neoproterozoic Ghaub Formation, Namibia: constraints on Marinoan glaciation.  Geology, v. 32, p. 817-820).  Their zircons come from a thin volcanic ash within isolated Neoproterozoic diamictites in central Namibia, and yield an age of 636±1 Ma.  Correlating the studied diamictites with the Namibian sequences elsewhere in the country relies on the presence of a supposed cap carbonate rather than lateral continuity.  The authors link them with the younger of the two Namibian diamictites, the Ghaub Formation, rather than the Chuos Formation that lies at depth, despite the fact that both well-studied units are sometimes overlain by carbonate sediments.  The conclusion is that the Ghaub is Marinoan, previously thought to be somewhere between 600 and 660 Ma.  Interestingly, the new occurrence of diamictites is divided vertically by two thick sequences of volcanic lavas, neither of which have been dated by the authors.

One of the leading experts on what actually constitutes incontrovertible evidence for glacial sedimentation is Nicholas Eyles of the University of Toronto.  He has become increasingly disenchanted with notions of Snowball conditions, on the basis of ambiguity in the very evidence said to signify them; diamictites with drop stones.  He and Nicole Januszczac have assembled a monumental paper that counsels caution, and perhaps more (Eyles, N. & Januszczac, N. 2004.  “Zipper-rift”: a tectonic model for Neoproterozoic glaciations during breakup of Rodinia after 750 Ma.  Earth-Science Reviews, v. 65, p. 1-73).  Part of their argument rests on the very lack of robust ages for Neoproterozoic diamictites that prevents believable correlations from continent to continent.  It is the globally synchronous nature assumed for these glaciations that gave rise to the “Snowball Earth” notion.  The palaeomagnetic latitudes are often used to support this, but they are error prone both palaeogeographically and geochronologically.  Accepting evidence for glaciation at low latitudes is no guarantee of support for even cold extremes, let alone an icebound world.  Solar heating in the Neoproterozoic was lower than now, and so, therefore, would be the elevations at which glaciers might form at different latitudes.  But the main problem is reconciling the features of many supposed glaciogenic diamictites with modern ideas of what truly constitutes evidence for glacial transport and deposition.  Few of the units on which the “Snowball Earth” hypothesis is based stand up to modern scrutiny.  Most of the diamictite packages occur in tectonically controlled basins, that were subject to episodic rifting.  Each can be considered to form the base of a “tectonostratigraphic” cycle, and many show abundant evidence of having formed as mass flows from a shelf into the basin.  They include olistostromes with huge rafts of carbonates likely to represent failure of carbonate platforms and huge submarine landslides, similar to those being discovered off many large islands today.  The 750 to 580 Ma period was one of the most dramatic episodes of continental break-up in Earth’s history as the Rodinia supercontinent was disassembled.  Continental uplift, resulting either from mantle plume activity or rebound of rift shoulders, could have resulted in large areas rising above the ice limit, even at low latitudes in those cooler times.  Those diamictites that are undoubtedly glaciogenic could easily have formed haphazardly in time.  The carbon isotope record of immense shifts in d13C during the Neoproterozoic, linked by some to repeated collapses and resurrections of life, might just as easily have occurred through efficient organic burial in active extensional basins and repeated major volcanism from plumes.  Only evidence of timing will tell, and three good dates for “Snowball Earth” events are simply not enough.

See also: Fanning, C.M & Link, P.K. 2004.  U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho.  Geology, v. 32, p. 881-884. Gives age of 709±5 Ma for tuff immediately beneath a supposed Sturtian diamictite. Also:  Calver, C.R. et al. 2004.  U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania. Geology, v. 32, p. 893-896.   Gives 575±3 Ma age for sills intruding a “Marinoan” diamictite, and 582±4 Ma for a rhyodacite immediately beneath it, similar to Gaskiers age above – worth a read later


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