News tips from UC Santa Cruz American Geophysical Union meeting -- December 15-19, 1996

For more information, contact Robert Irion at (408) 459-2495 or irion@ua.ucsc.edu

New way to gauge ages of stalactites may yield precise climate-change tool

Sunday, December 15, 2:45 p.m. Session OS72D, talk #6 Moscone Center, room 306

Speakers: Craig Lundstrom (408/459-4089 or cclund@bagnold.ucsc.edu) and Peter Holden (408/459-5559 or pholden@rupture.ucsc.edu)

The slow but relentless drippings of calcium-rich water in caves may open a new window on earth's past climate, thanks to a precise dating technique under development at UC Santa Cruz.

Preserved within the stark beauty of stalactites and stalagmites are two records of changes in the climate of the outside world. One such record is purely physical: When more water oozes from the roof of a cave, the rocky icicles grow faster. If researchers find similar growth spurts in caves over a wide region, they can infer that more rain fell during that period.

The second record relies on a well-known chemical phenomenon. As global temperatures warm up or cool down, and as precipitation patterns change, the ratio of "heavy" to "light" isotopes in rain or snow (defined by the weights of oxygen atoms in water molecules) changes in different regions. For instance, glaciers may lock up more light water, leaving heavier water in the sea. Paleoclimatologists can unmask past temperature fluctuations by detecting these subtle shifts in old layers of ice and in ocean sediments. The mineralized water that makes a stalactite also carries those clues, frozen in as the calcite deposit slowly lengthens.

It's no simple matter to retrieve those records in a useful way. Scientists must know when changes occurred in the growth rate or chemistry of a particular stalactite, but resolving those dates has proven devilishly difficult. The objects grow perhaps an inch every thousand years, making the sample sizes small. Further, traditional techniques for dating ancient geologic samples aren't precise enough to yield accurate ages for layers deposited within the last few thousand years.

That's the value of work by UCSC research specialist Peter Holden, postdoctoral researcher Craig Lundstrom, and undergraduate Andy Jacobson. The team adapted a technique used to date volcanic rocks with high precision. The method uses the radioactive decay of uranium, which stops at several elemental stations on its train ride of decay to lead. Two of those elements are thorium, which has a half-life of 75,000 years, and protactinium, with a half-life of 33,000 years. Others have used uranium-thorium analysis to try to measure the ages of cave deposits, but the addition of protactinium makes the technique much more sensitive.

The researchers analyzed a 10-inch stalactite that had fallen from the roof of a cave on California's central coast. They measured ages ranging from 8,500 years at the top of the stalactite to less than 800 years near its tip. The scientists estimate their technique is accurate to within 1 percent for the older end of the stalactite-- corresponding to a time window of about 80 years.

Paleoclimatologist James Zachos of UCSC hopes his group will be able to apply the method to resolve the frequency of terrestrial climate changes during the last 20,000 years on timescales ranging from decades to millennia. For example, cave deposits in the western U.S. and Mexico may reveal the broad effects of El Nino events on regional temperatures and precipitation.

Improved techniques needed for accurate measurements of trace metals in groundwater

Tuesday, December 17, 2:40 p.m. Session H22D, talk #5 Moscone Center, room 306

Speaker: Carol Creasey (408) 459-2088 or ccreasey@earthsci.ucsc.edu

Standard techniques for measuring metals in groundwater may drastically overestimate their levels, forcing owners of supposedly contaminated sites to "remediate" problems that may not exist.

Graduate student Carol Creasey and professor of earth sciences Russell Flegal, both of UC Santa Cruz, studied the groundwater from two wells at a reportedly contaminated site in central California. A consulting company also monitored the wells for trace-metal contamination using standard sampling and analytical techniques. The differences were striking: The concentrations of trace metals detected by the UCSC scientists ranged from 2 to 1,000 times lower than those reported by the consultants. In particular, the consultants claimed that the levels of cadmium and chromium in the groundwater exceeded the state's clean-water guidelines, whereas the UCSC tests revealed no such infractions.

Two key factors accounted for those discrepancies. First, the UCSC team used "low-flow" techniques to purge stagnant water from the wells and collect their samples, extracting less than one liter of water per minute. Pumping more quickly than that, Creasey says, can generate groundwater that is not representative of the natural system.

Second, Creasey and Flegal used rigorous "trace-metal clean" techniques at the site and during their laboratory analysis. These techniques prevent inadvertent contamination by ensuring that all materials touching the groundwater are thoroughly cleaned with acids and isolated from metallic objects or dusts. Also important, says Creasey, is to have more than one person assist with the water sampling so that samples are not contaminated.

Analysis of seven metals in the samples at UCSC showed that none of the concentrations exceeded federal or state standards, except for nickel in the most heavily impacted well. However, the consultants--who monitored the wells both five months before and one month after the UCSC team--found that levels of nickel, cadmium, and chromium violated the standards in one or both of the wells. For example, the consultants measured 51.4 parts per billion of chromium in one well (compared to a state maximum of 50 ppb), but the UCSC researchers detected just 0.0734 ppb, a 700-fold difference.

Creasey and Flegal acknowledge that not all consultants will have access to the precise analytical tools found in their UCSC lab. However, low-flow and trace-metal clean techniques are feasible for everyone, they say. Further, those techniques would raise sampling and analytical costs only moderately.

"Consultants should try these methods to see whether they make a difference," Creasey says. "Extensive monitoring and remediation of a site that may not be contaminated is certainly more costly than conducting the measurements right the first time with more careful techniques."

Roots of "hot spots" may extend to earth's core-mantle boundary

Tuesday, December 17, 3:20 and 4:25 p.m. Session T22E, talks #6 and #10 Moscone Center, room 307

Speakers: Edward Garnero (408/459-5139 or eddie@rupture.ucsc.edu) and Quentin Williams (408/459-3132 or quentw@rupture.ucsc.edu)

"Hot spots," the isolated patches of volcanism unrelated to plate tectonics, may spring from surprisingly deep within the planet: the turbulent boundary between earth's mantle and its core.

That conclusion, sure to put scientists on the spot at the AGU meeting, has arisen from intense study of a layer at the base of the mantle that apparently contains partially molten rock. Researchers at UC Santa Cruz and elsewhere analyzed a peculiar type of seismic wave that skims along the mantle side of the sharp core-mantle boundary. In several regions, something bogs down the speed of the waves by about 10 percent--a huge amount by geophysical standards. The most likely cause, the researchers maintain, is that a small fraction of melted material bathes the mantle rock and transforms it into a thick mush.

Seismologist Edward Garnero of UCSC has worked with Donald Helmberger of Caltech and UCSC's Justin Revenaugh to characterize the layer. So far, the seismologists have probed about 45 percent of the core-mantle boundary in search of the partial melt. Temperature constraints at the boundary would require that if the layer exists anywhere, it would encircle the globe. However, the layer appears substantially thicker in some regions--most notably under the south-central Pacific, Iceland, East Africa, and the Azores. The seismic evidence thus far points to a layer of partial melt no thicker than about 20 kilometers in those zones. Elsewhere, it would be much thinner--5 kilometers or less--and would evade seismic detection entirely.

These first few swaths of partial melt, notes UCSC mineral physicist Quentin Williams, lie 2,900 kilometers beneath some of the most well-known hot spots on the planet. That correlation is not yet perfect, but it is no coincidence, he believes. "This may be the smoking gun that hot spots originate from the core-mantle boundary," Williams says. Although others have proposed such a deep genesis for hot spots, no one has yet unveiled solid evidence to support that theory.

Williams envisions a partially molten layer that becomes unstable at irregular intervals above the core-mantle boundary, welling up into thicker, stubby plumes. These zones would help heat flow with great dispatch out of the liquid outer core and into the lower mantle. "This may be the first image we have of a process that spans the mantle," Williams says. "Feeder zones of plumes that start at the base of the mantle would rise all the way to the surface and create hot spots."

This line of reasoning has several fascinating implications. For instance, if there is indeed a partially molten layer at the base of the mantle, it would be about a trillion times less viscous than the solid mantle immediately above. "That's geodynamically very interesting," Williams observes. Further, the scenario raises a "chicken and egg" conundrum: Would upwellings in the partially molten layer trigger and sustain the hot spots, or would hot spots draw material up through the mantle and cause the upwellings? Stay tuned as the debate heats up.

Recent quakes don't appear to violate seismic gap hypothesis

Thursday, December 19, 8:30 a.m.-noon Session S41A, poster #16 Moscone Center, Hall D

Presenter: Susan Schwartz (408) 459-3133 or sschwartz@earthsci.ucsc.edu

The seismic gap hypothesis of earthquake recurrence, threatened by four large and seemingly "premature" quakes in the last two years, appears under closer scrutiny to remain valid.

The hypothesis maintains that once an earthquake ruptures a fault, the same region will not rupture again until enough time passes--usually many decades or centuries--for stress to rebuild. Researchers have used this hypothesis to look for "gaps" in fault zones that have a previous history of earthquakes but have not ruptured recently, and to identify those gaps as the most likely sites for quakes in the near future. This notion is commonly applied around the Pacific Rim, where most of the world's great earthquakes occur.

However, four events in the last two years seemed to cast doubt on this approach. In each case, quakes slightly smaller than magnitude 8 struck in areas where even larger earthquakes had occurred within the last 10 to 30 years. That's not nearly enough time in most cases, seismologists believe, for such levels of stress to reaccumulate in a fault zone.

Susan Schwartz, director of the W. M. Keck Seismological Laboratory at UC Santa Cruz, decided to take a closer look at the details of each set of earthquakes. Since the early 1960s, global seismic records have allowed researchers to reconstruct how a quake occurs--where the earth slips along the fault and how that slip is distributed. "We've found that earthquake slip along a fault can vary dramatically," Schwartz says. "Some regions of the fault zone move a lot, while others may not move much at all, even if they're nearby."

In three of the four cases, Schwartz found, the recent quakes broke sections of the faults that had moved little in the original events. Those quake pairs were in the Kuril Islands north of Japan (M 8.5 in 1963, M 7.9 in 1995); the Solomon Islands east of New Guinea (M 8.0 in 1971, M 7.8 in 1995); and northern Honshu in Japan (M 8.2 in 1968, M 7.7 in 1994). "These are all nice examples of subsequent earthquakes filling in the gaps," Schwartz says. "They are not repeat events, even though the epicenters are close together and the aftershock zones have some overlap."

The fourth set of earthquakes, in the central Aleutian Islands, is harder to unravel. There, a M 8.6 quake in 1957 was followed by a M 8.2 in 1986 and a M 7.9 in 1996. The slip distribution for the 1957 quake is difficult to determine, Schwartz says, because it predated the global seismic network. At the very least, it appears that the 1986 and 1996 rupture zones were markedly different.

"The physical concept behind the seismic gap hypothesis is still correct," Schwartz says. "If an area slips in an earthquake, the same area should not slip again in the near future. But because the slip is not uniform over the whole rupture area, there is no 'characteristic' earthquake to identify from one cycle to the next." The need to tease out such levels of detail about each rupture, she says, may make it difficult for seismologists to apply the seismic gap hypothesis in a meaningful way.

Fluids flow fleetly under the seafloor

Thursday, December 19, 1:35 p.m. Session T42C, talk #1 Moscone Center, room 130

Speaker: Andrew Fisher (408) 459-5598 or afisher@earthsci.ucsc.edu

Hydrogeologists have taken the closest look yet at the intricate cycle of fluids that flow relentlessly beneath the seafloor. That flow, it now appears, is far more forceful than expected.

Driven by the heat of the planet's interior, water courses through pores and cracks under the ocean in earth's upper crust. The water leaches minerals as it flows, altering the crust and suffusing the ocean with important elements. This process occurs dramatically at "black smokers" and other hot vents along volcanic midocean ridges. It happens to a far greater extent along the sweeping flanks of these ridges, and possibly under most of the rest of the ocean as well. But because the seafloor is so remote, scientists have known precious little about the details.

That all changed during Leg 168 of the Ocean Drilling Program, a two-month cruise in July and August off the Pacific Northwest. An international team of researchers, led by cochief scientists Andrew Fisher of UC Santa Cruz and Earl Davis of the Geological Survey of Canada, drilled as far as 1,900 feet into the seafloor on the flanks of the Juan de Fuca Ridge, where the planet churns out fresh slabs of oceanic crust.

The researchers collected the first known samples of pristine "basement" water--fluids trapped under the seafloor for many thousands of years. Chemical analysis of these and other samples points to a surprisingly energetic cycling of fluids and heat beneath the sediments that shroud the young crust.

"There's no question the water is moving much faster than we thought," Fisher says. The time it takes for the entire volume of the world's oceans to cycle through these subseafloor systems, he notes, could be "much, much shorter" than the previous estimate of one to two million years.

Fisher and his colleagues found that the upper oceanic crust beneath seafloor ridges is "overpressured," meaning that hot water surged from their drill holes. Tests also showed that the crust is most conducive to fluid flow within layers that are perhaps just 10 meters thick. "We're still hard-pressed to explain how these fluids can travel laterally within the crust for 50 to 80 kilometers, but the thermal and chemical data suggest this is exactly what happens," Fisher says.

Cochief scientist Davis adds: "We've come to realize that this kind of water circulation is much more important than we believed just a few years ago. We suspect these processes take place within a large part of the seafloor, perhaps most of it, and play important roles in forming mineral deposits and in helping to control the composition of seawater."

The Leg 168 scientists installed deep-sea observatories, called CORKs, in four of the drill holes. The CORKS will record the pressures and temperatures of fluids with exquisite sensitivity for several years; manned or robotic vessels will retrieve the data. Fisher and Davis expect the results will help paint a complete picture of the forces that impel the fluids.

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