California Droughts And Carbonate Cycles

When I was a graduate student in the mid 1990's one of the hot research topics in carbonate sedimentology was the study of shallow marine carbonate depositional cycles. Sediments in shallow marine and even deeper environments  are deposited in packages or cycles during periodic sea-level rise and fall. There is a rhythmic beat to this, regulated by the waxing and waning of polar ice sheets or by periodic tectonic subsidence or by an internal clock beating to changing currents and sediment distribution patterns. The periodicity of these cycles is on the scale of tens of thousands of years.

Which of these mechanisms is dominant? That was the big question and the big fight.

GSA meetings used to be fun listening to rival research groups intellectually slug it out. Over the years the waning and waxing of ice sheets (allocyclicity) has been recognized as a common mechanism of generating sediment cyclicity but to give other processes their due, tectonic subsidence and autocyclicity i.e internal forcing due to self regulated sediment accumulation and distribution patterns are also recognized from specific environments.

We worry about a lot of other problems these days, global warming and its effects of rainfall being one. Scientists involved earlier in their career in the study of marine carbonate cycles have started paying attention to geological beats taking place on a much higher frequency.

Isabel Montanez who has published a lot on marine carbonate cycles has a paper with graduate student and colleagues on the rhythmic precipitation of carbonate minerals within cave stalagmites from the Sierra Nevada mountains of California.

Late Pleistocene California droughts during deglaciation and Arctic warming - Earth and Planetary Science Letters

And here is the Science Daily press release.

Cave stalagmites form by minerals precipitating from water dripping from the roofs of caves. Growth speeds up during the rainy season and slows or stops during the dry periods. Growth bands can thus reflect yearly growth much like tree rings. This allows scientists to resolve changes taking place over tens of years. One fairly long stalagmite can record growth over hundreds to few thousand years. In this study the scientists have a record of stalagmite growth - radiometrically calibrated - over a time interval from about 16.5 thousand to about 8.8 thousand years ago covering the end of the last ice age.

During this time interval the climate record from Greenland indicates alternating warm and cold periods lasting a few hundred to a few thousand years.  In California the chemical composition of the stalagmite measured over the same time period indicates that during the Greenland warm periods California became drier while Greenland cold periods corresponded with a wetter California.

How do you find this out? Of major interest are the variations in the oxygen isotopes and to a lesser extent the carbon isotope composition of the calcite in the stalagmite.

A graphic makes the geochemical life of oxygen isotopes easier to conceptualize.

Source: SAHRA

The ratio of two oxygen isotopes  ¹⁸O and ¹⁶O are measured. The notation d¹⁸O  refers to the ratio of  ¹⁸O to ¹⁶O. The more negative value of d¹⁸O   means enrichment in ¹⁶O. ¹⁶O being the lighter isotope is preferentially enriched in the vapor phase. So if it rains only a little then the precipitation is going to be enriched in ¹⁸O. If it rains a lot i.e. if most of the moisture in the air condenses then eventually the rains will become more and more enriched in ¹⁶O.

This is called the rain out or amount effect.

This oxygen in the rain eventually ends up in the mineral calcite precipitating on the floor of the cave. In the graphic above substitute initial precipitation and later precipitation taking place in different places to less rain and more rain taking place at the same location. 

Drier conditions will result in the mineral calcite that makes up the stalagmite growing from relatively ¹⁸O enriched water (elevated d¹⁸O ) and wetter periods will result in growth from relatively ¹⁶O enriched waters (depleted d¹⁸O).   These changing values are encoded in the calcite growth bands of the stalagmite. The scientists have found with disturbing implications for future California climate that in the past, a decline of ice sheets and warmer climate resulted in a drier California. History is giving us a warning about the expected rainfall patterns over California.

A quick note on carbon isotopes. Here two isotopes ¹³C and ¹²C are measured. Organic carbon is enriched in the lighter isotope because plants preferentially take up the lighter isotope during photosynthesis. During wet periods when the ground above the cave roof supports a lot of vegetation, the water seeping through the vegetation will incorporate a lot of the lighter isotope. During dry periods the water seeping through the cave roof will have relatively less of the lighter isotope. So dry periods will have an elevated d¹³C value (ratio of ¹³C and ¹²C).

So rhythmic deposition or precipitation of carbonate cycles over a wide range of time scales gives us different types of information about earth processes.

Cycles deposited over tens of thousands of years in marine basins tell us about either external controls on sea-level rise and fall like the periodic changes in the earth axial tilt or orbital parameters. Or they tell us about local tectonics changes or other internal regulating mechanisms.

On a decadal scale, precipitation cycles of calcite in stalagmites, may tell us about changing rainfall patterns and warn us about future droughts and water shortages.

Occasionally they may even give us insights into social and cultural upheavals such as the rise and fall of Chinese dynasties. Check out this finding. Its uses the same isotope principles from cave deposits to unravel Chinese climate history over the last thousand years or so and its impact on the Tang, Yuan and Ming dynasties in China. 

None of these press releases though have actually explained the isotope effect.

Which kind of irritated me and prompted this post.