Last year, when Moberg was published, I pointed out witha slightly arched eyebrow that one of the two most important contributors to any 20th century HS-ness in Moberg was the increasing percentage of subpolar foraminifera (G. Bulloides) in the Arabian Sea – intuitively not a direct indicator of warming. Having visited the foraminifera literature in more detail in my reading of Hansen (who is more interested in G. ruber), I noticed some pertinent discussion of G. Bulloides in the Arabian Sea, which confirmed my initial reading. G bulloides percentage directly indicates upwelling cold water, which in turn is linked to a stronger winds (i.e. stronger monsoon), which is said to be associated with warming onshore.
Moberg stated that proxies were chosen according to the following policy:
Records that have been interpreted as reflecting annual mean temperatures were preferred, but a strict application of such a criterion would have led to a too limited amount of data. Therefore, data representing seasonal (mainly summer) temperatures were also allowed.
Here is Moberg’s diagram of the percentage G. Bulloides is shown in Figure 1, with his description in the caption (my bold),
Figure 1. Percentage of G Bulloides in an Arabian Sea core. Original description: A combination of two marine sediment records from the Arabian Sea [Gupta et al 2003; Overpeck et al 1996] in which the percentage of the foraminifera Globigerina bulloides reflects the extent of ocean up-welling, which is determined by the strength of monsoons, which in turn indirectly reflect both summer and winter large-scale temperature changes through the differential seasonal heating and cooling of the Asian continent and surrounding oceans [Overpeck et al 1996]. We used data from Core 723A [Gupta et al 2003;] for the early years up to 1390 A.D. and data from Core RC2730 [Anderson et al 2002] from 1391 to 1986 A.D. (c.f. Fig. 3c in ref. 15). Although this record reflects temperatures only indirectly, it was included to improve the balance in the geographical distribution of proxy sites.
Conan and Brummer 1999 have an interesting discussion of foraminera in a nearby location – offshore Somalia at about 11N; 52E in an upwelling zone. They studied both foraminifera both in a trap at 1265m and coretop at 1625m. Figure 2 below shows the annual cycle of sea surface temperature at the sites – with a remarkable drop in SST during the summer upwelling.
Fig. 2. Sea surface temperature at the mooring site (upper panel (a)) for 1992}1993 and fluxes of particles ’100 lm to the sediment trap at 1265 m water depth, 268 m above the bottom (lower panel (b)). The large circles are in situ measured, small circles are derived from remote sensing and squares are monthly averages (see Brummer et al., 2000). Please note that particle fluxes are displaced by 2 weeks to account for the delay in shell production and settling of the planktic foraminifera. Note the different scale at right for the `unidenti”ed fragmentsa category which predominantly consists of bioclastic grains.
The next figure shows the seasonal distribution of foraminifera. You will notice that G bulloides is associated with cold upwelling water.
Conan and Brummer Fig. 5. Species composition (%) of planktic foraminifera’100 lm (a) and’125 lm (b) of sediment trap MST8-B.
Finally the last figure shows the seasonal distribution of shell production – as annual yields are obviously integral/
Conan and Brummer Fig. 3. Seasonal shell fluxes of planktic foraminifera (shells’100, ’125, ’150 lm m~2 d~1) intercepted by sediment trap MST8-B at 1265 depth, 268 m above the sea #oor in the Somalia Basin from June 7,1992 to February 14, 1993. The peak flux observed in the 2 samples of October in the’100 lm fraction is probably caused by resuspension of bottom sediment material from the shelf and upper slope but did not affect the’125 lm fractions.
I’ve also mentioned before that the values of this proxy are nowhere near being normal – they are more like a uniform distribution. Despite the assertions that the proxies were selected to “reflect” temperature, the reflection here is through a very distorted mirror. Additionally, although the proxy increases are interpreted to show increased warmth, they do not reflect warmth in the area of the proxies but hypothesized warmth elsewhere as a result of increased winds. To the extent that Yang’s China composite draws on this onshore area, isn’t this already counted?
S.M.-H. Conan, G.-J.A. Brummer 1999. Fluxes of planktic foraminifera in response to monsoonal upwelling on the Somalia Basin margin Deep-Sea Research II 47 (2000) 2207}2227
Gupta, A. K., Anderson, D. M. & Overpeck, J. T. Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean. Nature 421, 354-357 (2003).
Overpeck, J., Anderson, D., Trumbore, S. & Prell, W. The southwest Indian Monsoon over the last 18 000 years. Clim. Dyn. 12, 213-225 (1996).
Anderson, D. M., Overpeck, J. T. & Gupta, A. K. Increase in the Asian SW Monsoon During the Past Four Centuries. Science 297, 596-599 (2002).