John A and Steve M,

there’s a lot of arm waving involved when interpreting d18O signals in natural archives. This is because in most cases the system is under-determined in terms of the number of independent parameters measured.

The foraminiferal data in Cariaco is a good example. The carbonate oxygen isotope thermometer describes the partitioning of oxygen-18 between water and carbonate as a function of precipitation temperature. i.e. If you know the water d18O and the carbonate d18O an assessment of precipitation temperature can be made.

In most studies the water d18O is not known and therefore it is not possible to make an accurate assessment of temperature. Therefore the interpretation is wrapped up in caveats such as:

“The long-term trend in the Globigerinoides ruber d18O record can be explained by two different but equally plausible scenarios. First, the increase in d18O may indicate that tropical summer-fall SSTs have cooled by as much as 2C over the last 2000 years, possibly as a result of a long-term increase in upwelling intensity. Alternately, comparisons to other studies of ITCZ and regional evaporation/precipitation variability suggest that much of the d18O record is influenced by decadal- to centennial-scale variations in the mean annual position of the ITCZ and associated rainfall patterns.”

Tis simply states they can’t unravel the effects of temperature or salinity changes.

If one accepts that the Mg/Ca signal in forams is dependent on temperature, then one can try to unravel salinity changes using the d18O data.

With respect to Lonnie Thompson and ice core we’re looking at a different temperature effect. Here Thompson argues that the precipitation temperature is recorded in the d18O of the ice. This is a well known effect in mid-latitudes where the average annual precipitation composition is closely correlated with local temperature. However, it is not the case at all sites, notably low latitudes and in monsoonal systems where an amount effect dominates temperature. Ideally, extensive studies of local precipitation, temperature, weather systems need to be completed in combination with ice core records in order to understand or calibrate the d18O-temperature relationship. This is very rarely done and is a great problem in most studies of this type.

Steve, you also raised the point anout forams, d18O and ice ages. Here there is a large change in foram d18O composition between ice ages and interglacials. This is not a temperature signal. Rather the forams are recording the increasing d18O composition of the global ocean as isotopically light water is locked up in large ice sheets. Of course this signal is cocordant with temperature proxies on the ice age-interglacial time scale.

Finally, one may legitimately ask what use is d18O if it can’t record temperature. The answer is that it can given appropriate systems. One is speleothems where it is possible to measure both drip water composition from fluid inclusions and the carbonate composition. The work is difficult and only a few studies have been completed to date. Another is that there are new techniques in oxygen isotope chemistry being developed that have the promise of being a true thermodynamic thermometer. These measure the partitioning of 18O and 13C within a single carbonate phase, that is the preferential binding of 18O to 13C which is temperature dependent.

The Little Ice Age (LIA) is characterized by a pronounced 1.5C SST decrease between A.D. 1500 and 1640, with a particularly steep drop between approximately 1630 and 1640, almost exactly coincident with the beginning of the Maunder Minimum in 1645

Steve and the group. In regards to high resolution studies of the transition into the Maunder minimum: Are there other records of sufficient resolution regarding this transition period?

Great site. Thanks Steve M. and all your helpers and excellent posters.

You couldn’t make that circa 1878 could you? Then it matches a spike in HADCRUT3 NH.

JF

I can’t parse that.

The long-term trend in the Globigerinoides ruber d18O record can be explained by two different but equally plausible scenarios. First, the increase in d18O may indicate that tropical summer-fall SSTs have cooled by as much as 2C over the last 2000 years, possibly as a result of a long-term increase in upwelling intensity. Alternately, comparisons to other studies of ITCZ and regional evaporation/precipitation variability suggest that much of the d18O record is influenced by decadal- to centennial-scale variations in the mean annual position of the ITCZ and associated rainfall patterns.

I have no particular objection to people interpreting lower dO18 values, such as this, in terms of these sort of explanations. My objection lies not with this interpretation, but with the people such as Lonnie Thompson who will seize on upticks in dO18 as evidence of global warming (e.g. the Hockey Stick in Inconvenient Truth.) If dO18 doesn’t work consistently, you can’t just pick series where it goes up.