Barker et al. [Science 2001] studied two glacier-fed tarns (micro-lakes) high on Mount Kenya -, Small Hall Tarn (SHT) at 4289 m and Simba Tarn (ST) at 4595 m.
They noted that prior studies [ Rietti-Shati et al, Science 1998; Karlen et al, Ambio 1999] on Hausberg Tarn (4370 m) on Mont Kenya had shown the following:
A 3000-year d18O diatom record obtained from Hausberg Tarn [4370 m above sea level (asl)], a glacier-fed lake on the northwest flank of Mt. Kenya (5), contained short-term minima in d18Odiatom attributed to glacier melting and longer term minima attributed to increased water temperatures (5, 6), in contrast to the positive relation assumed between d18O and air temperature in ice-core studies (7).
They describe local precipitation sources as follows (which is presumably similar to Mount Kilimanjaro, Thompson not providing any information):
Precipitation in the summit zone of Mt. Kenya falls mainly from March to May and in October and November, although some occurs in every month. Total precipitation declines with altitude above the tree line (~3000 m asl) and is <900 mm -1 above 4500 m asl, where it mainly falls as snow (3). The main moisture source is the southwest Indian Ocean, because of southeasterly airflow in the lower troposphere during the rainy seasons (3, 8). Mean annual temperatures are close to freezing at both sites (1.3°C at SHT, -0.7°C at ST), with a monthly variability of 1-2°C but a large diurnal range (10° to 20°C) (3).The cores show contrasts in dO18. In periods of elevated dO18 (zones 1, 3, 5, 7) diatom-species richness and green algae levels were the highest. contrasting periods with negative dO18 excursions occur which are accompanied by high magnetic susceptibility values. The diatom assemblages are dominated by Fragliaria and the high magnetic contents evidence allochthonous sediment inflow.
The synchronous timing of the negative diatom dO18 excursions and the high magnetic susceptibility are shown below:
Detail from Barker et al. Figure 1.
They focus on the negative excursions at several periods as shown in the graph below.
Barker et al. Fig. 3. Isotope data from Simba Tarn, Small Hall Tarn, and Hausberg Tarn (5). These data are compared to lake levels in the Ziway-Shala Basin, Ethiopia, derived from 14C-dated shorelines (32-Street-Perrott, Nature 1990).
The similarity of the ST, SHT, and Hausberg Tarn isotope curves indicates underlying climate forcing modified by local hydrological factors. At tropical stations, the d18O values of monthly rainfall (d18Oprecip) exhibit a far stronger stronger correlation with total precipitation (the “amount effect”‘?) than with air temperature (14, 15).
They point out that at the LGM, the gradient between the ocean and the continent was reduced, leading to a weaker monsoon and reduced "amount effect", thus contributing to the "high values of dO18 (maximum +36.7 per mil) recorded on Mt Kenya in the late glacial period (14 to 11.2 ka).". (Note that high dO18 in glacial periods is opposite to a Thompson view of the world.) Barker et al.:
Unfortunately, the modern isotopic composition of precipitation on Mt. Kenya is unknown, apart from spot dry-season measurements measurements of ; 10 to 12 per mil at 4200 m asl (5). However, d18Oprecip values at Kericho (2130 m asl) and Muguga (2070 m asl) in the Kenya Highlands are ~ -4.4 and -3.8 per mil, respectively (14), the most negative isotopic values occurring during the wet seasons. The monthly d18Oprecip values for these stations (22) lie close to the global meteoric line, indicating that precipitation formation occurs under isotopic-equilibrium conditions, with minimal evaporation of rainfall (23). For d18O, the impact of orographic uplift is ~22.7 to 22.9 per mil km-1 in East Africa (24). This suggests that d18O precip at our two study sites averages around 210 to 211.5 per mil. These estimates are compatible with spot measurements of -9 per mil on river runoff from 4000 m asl (24), -8 per mil on surface snow from the Lewis Glacier (4), -6 to -7 per mil on Hausberg and Oblong Tarns (open lakes fed by glacial meltwater), and 11 per mil on Naro Moru Tarn (a closed lake) (5).
During wetter intervals in the past, the mean isotopic values of lake water would have been lowered by increases in precipitation (14, 16, 25) and cloud height [giving lower cloud-top temperatures (23, 26)], strongly reinforced by decreased evaporation and possibly by lake overflow (in the case of SHT).
A comparison of the Holocene variations in d18Odiatom (Fig. 3) with alkenone SST estimates for the southwest tropical Indian Ocean, the source of much of the precipitation on Mt. Kenya, shows that the isotopic minima around 9 ka and at 6.9 to 5.8 ka corresponded to high SSTs (27 – Sonzogni et al, QSR 1998 ).
At present, heavy seasonal rainfall totals over the eastern Kenya Highlands are strongly linked to positive SST anomalies over the tropical South Atlantic and Indian Oceans, as well as to El Nino-Southern Oscillation events (28). The heavy rains of 1961-1962 provide a modern analog for the abrupt, high-amplitude d18O diatom minima. In November 1961, for example, the precipitation on the footslopes of Mt. Kenya exceeded 275% of normal (28) as a result of onshore flow from a large area of anomalously warm SSTs in the western Indian Ocean. Evaporation was also greatly reduced by dense cloud (29).
Our data suggest that anomalously heavy snowfall on the peaks of Mt. Kenya may contribute to the neoglacial ice advances dated ~5.7 ka, 3.2 to 2.3 ka, and 1.3 to 1.2 ka (6).
What does this mean for Thompson’s interpretation of Mount Kilimanjaro? First, it seems to me that Thompson’s arm-waving identification of the negative dO18 excursions at Mount Kilimanjaro with solar minima and, in particular, his identification of the Wolf Minimum with excursion #2 are very speculative at best. On the other hand, the existence of these glacial tarns through the Holocene certainly suggests that glacier-fed tarns are a recurrent feature in the Holocene, arguing both for the existence of glaciers (either more or less continuously or at least periodically.) Thompson’s year 4200 event which he highlighted does not appear at Mount Kenya – why is that? It’s frustrating that Thompson, the later article, doesn’t bother reconciling to Mount Kenya.
The negative excursions are surprisingly episodic- why is that? Since the negative excursions are associated with high turbidity, it looks like they are associated with periods of glacial melt (as well as with high snowfall – the negative dO18). I can think of two contrasting opposite states: no glacier and no melt; and a frozen glacier and no melt. Would it be possible that there are episodic periods of very high snowfall creating transient glaciers – with the long periods of high dO18 in the tarns representing absent glaciers? What accounts for the two excursions around ~6000 BP?
P. A. Barker, F. A. Street-Perrott, M. J. Leng, P. B. Greenwood, D. L. Swain, R. A. Perrott, R. J. Telford, K. J. Ficken, 2001, A 14,000-Year Oxygen Isotope Record from Diatom Silica in Two Alpine Lakes on Mt. Kenya, Science 292, 2307-2310