Examine how the ‘mutual climatic range’ method, and its recent developments, is used to reconstruct past temperatures.
The Mutual Climatic Range (MCR) method is used in the reconstruction of the palaeoclimatic conditions of geographical areas based on the assumption that if the present climate extent of an indicator species is known, then fossil analogues of that species will have had the same tolerance range and therefore act as a proxy of paleoclimates that were of that tolerance range. There is also the idea that multiple species found in assemblages together will have had overlapping tolerance ranges and graphing the ranges together will produce a more precise model of the palaeoclimate of the area in which they occurred. The method stems from work carried out by Iversen in 1944 in which the first attempted was made to reconstruct past climates by comparing the modern climatic ranges of plant pollens that were collected from sediments. The modern technique was further developed by Atkinson and others for beetles, although there have been some additions to the method, the basic principles still stand.
The method works by plotting the maximum July temperature of a climatic space against the temperature range between the July maximum and the January minimum temperature: Tmax-Tmin= Trange. The method makes three assumptions:
1. Climatic tolerances of modern beetles can be adequately defined.
2. Climatic preferences of individual species have not altered significantly throughout the Quaternary.
3. Temperature is the main determinant of the gross geographical distribution of most modern beetle species (Bray et. al. 2006).
The second assumption is validated by evidence from isotope records in Greenland which shows that fossilized beetles had altered their range in the past as the climatic conditions no longer became viable for them. The distribution of each species in an assemblage is then converted into a climatic envelope in which the species occupies at present day, this is the species climate range (SCR).
The method is a useful tool because it avoids subjective interpretation and possible bias. Furthermore, geographical range limits are broad and cannot take factors such as altitude, microclimatic variation etc. into account. The MCR method ignores geographical location and only focuses on climatic parameters.
Fig 1. Shows the hypothetical thermal envelopes for two species, displaying their mutual climatic range
Coleoptera are the main indicator species used for the method because they are widespread globally. There are certain species with more niche climatic envelopes as they are stenothermal which means they hold more climatic significance. In addition they respond quickly to changes in climate and do not suffer distortion effects by the wind or water (unlike pollen or ostracoda). However, only carnivorous and scavenger beetles may be used for the method because herbivorous species will most likely only show the distributions of the vegetation they eat.
Coope and Lemdahl (1995) used the MCR method to reconstruct paleoclimates in N. Europe (Britain and Sweden). From the British records, it was evident that there was a very rapid rise in summer temperatures 13,000 years BP. The Swedish records indicated a more gradual rise in temperatures, most likely due to cooling from the Scandinavian ice sheet.
Lemdahl (2000) then used the MCR method to reconstruct late glacial to early Holocene climate changes with beetle assemblages taken from cores in Krakenes Lake, Western Norway. 36 beetle taxa were identified from the sediment however this yielded sparse records, which indicates low diversity in the environment during the Younger Dryas. The MCR suggested colder, more continental conditions pre Younger Dryas with a Tmax of 10 degrees celsius and a Tmin of -12 degrees celsius. The low diversity of the Younger Dryas was shown to increased rapidly into the Holocene.
There have been developments with the method e.g the maximum likelihood estimation which increases the quality of the available coleoptera data. In addition it lowers the error rates in estimating past climates, producing higher quality climate models. It is effective for places with lower taxonomic data on modern beetle species. It plots the modern species using a GIS and climatic surfaces so a climatic range can be identified for each species (using mean February temp and mean minimum July temperature). In addition with later work, local climate statistics such as mean annual temperature started to be taken from meteorological stations in the overlap zones of study to achieve more accurate data for the SCRs.
The method has also been used for other proxy groups such as ostracoda, molluscs and pollen. The most notable is the work with ostracoda by Horne (2007). The hydrochemistry and hydrology of lakes is affected by abiotic factors such as precipitation rates and the geology of the surrounding area, these factors have a major influence on ostracod populations and mean that they are a good candidate for past climate analysis. The mutual ostracod temperature range (MOTR) has the same assumptions as those of the MCR method for coleoptera and uses three components: A GIS, modern climate data and non-marine ostracod data. The MOTR uses GIS to make estimates of the climate parameters that best fit the geographic distribution of the individual species that are being used. The results can be supported by measuring the stable isotope and trace element content of the fossilized ostracod valves as these give an indication of the environment that they were living in. Temperature can be an issue in this instance as the relationship between water temperature and air temperature is complex, however, Mezquita et. al. (2005) avoided the problem by taking the mean Jan and July air temperatures from GIS models. A criticism of this technique however is, as previously noted, the fact that ostracods may be subject to distortion and reworking effects within the water.
The use of the MCR for plant matter has also been controversial because it is likely to come under disturbance and redistribution by the wind and because there is a 100 to 300 year lag in their redistribution patterns in response to climate and they are difficult to identify at species level which make them a less reliable proxy to work with than faunal species.
There are some limitations that come with the use of MCR. Firstly, the approach only takes account of the presence or absence of the indicator species that is recorded, Huppert and Solow (2004) argue that, as a result, the link cannot be tested using conventional statistical methods. This may however be improved by ubiquity analysis which seeks to better constrain uncertainties in MCR estimates. (Bray et al 2006) There is also the risk that false positives may be obtained even with improved climatic data, in which a species may be assigned a climate that is not representative of their actual preference e.g in areas with a steep climatic gradients, some species may live in microclimates that are away from the norm. In addition, the range of modern data for some species is sparse and it can never be certain if populations have reached their full climatic range or that the distributions of fossil species were the same as they are today. Finally some species may have temporarily coexisted during transitional phases leading to the mixing of fossils that would not typically be found together.
The MCR approach is good for use with faunal indicator species, particularly coleopteran and it allows comparison with other methods of Quaternary climate reconstruction. The method has made significant developments since its initial creation which has improved the accuracy and reliability of the results it yields. It is also very likely that future work will further improve this approach, particularly with the aim of obtaining better results for molluscs and ostracods.