Correlation of Fluvial Sequences in the Mediterranean by Macklin, Fuller, Lewin, Maas - Essay Example

? Review The research paper d “Correlation of fluvial sequences in the Mediterranean basin over the last 20 ka and their relationship to climatechange” by the authors M.G. Macklin, I.C. Fuller, Lewin, G.S. Maas, D.G. Passmore, Rose, J.C. Woodward, S. Black, R.H.B. Hamlin and J.S. Rowan, offers a critical review of how climate changes over past years have impacted upon the river basins in the Mediterranean region. This review will address the value of the content of this research paper and its contribution to the impact of climate induced changes. The research paper commences with an introduction. The next section details the eleven catchments that formed the basis of the study and the alluvial geochronologies that are associated with each of these catchment areas. The next section of the paper describes the process for the development of the geochronology of river alleviation in the Mediterranean basin through the plotting of the ages and confidence intervals of the dated alluvial units, in order to determine the critical relationships existing between the various basins. The last segment of the research paper arrives at conclusions based upon the results obtained from the research study and the research priorities for the future. This research paper has a two fold objective; first to construct a provisional correlation of alluvial sequences during the late and middle Pleistocene phases of river alluviation in the Mediterranean basin; a correlation that can also be tested and second, to evaluate the responses of the rivers to the climate changes that have occurred over the past 200,000 years in the Mediterranean basin. In order to achieve these objectives, the research paper first of all points the difficulties that have traditionally been associated with the correlation of alluvial deposits due to the problems in accurately dating those deposits that are older than the 14C method. During the 1990s several other methods were devised to examine alluvial sequences, including geomorphological, pedostratigraphic , sedimentological and geochronological studies, of which the study of alluvial geochronologies received an impetus through the successful use of sediment based luminescence techniques for the dating of fine grained inorganic fluvial deposits. The authors underline the importegion, whance of the development because it enabled a determination of river erosion and alleviation phases of the late and middle Pleistocene periods. These phases could then be compared with moderate resolution records of climate change in the Mediterranean region and high resolution records of climate change in the north Atlantic regions. The objective of the authors in the study is to present a new correlation of the late and middle Pleistocene alluvial sequences in the Mediterranean region. Rather than considering all available data on alluvial units, because earlier studies have shown that these sedimentary sequences were poorly dated, the authors have chosen to examine those river basins where the geomorphic development is relatively well understood and dating is not based primarily based on the 14C dating method. Eleven catchment areas have been identified in the Mediterranean region, which meet the strict geomorphic and geochronological criteria that were identified for inclusion in the study. Two notable areas where securely dated Pleistocene alluvial deposits were not available were Italy and the Mahgreb region of north Africa. The catchment areas that were selected by the authors in the study were however representative of the full range of river environments that are found in the Mediterranean region, namely four alluvial types, i.e., (a) extensive terraces formed by aggradation in episodes (b) quasi continuous incision (c)medium sized basins where the headwaters supported glaciers and(d) alluvial fans or limestone catchments. The authors have discussed the two techniques of dating, one is luminescence dating, wherein by zeroing in on luminescence signals, it is possible to determine the age of a depositional event. While this can pose problems in some environments, high light levels in the Mediterranean region make this a viable method to use. Another method that may be used is uranium series dating involves the dating of post depositional calcium carbonate cements, which also provides a minimum age of a fluvial depositional event. This method is related to climate, because the formation of calcretes is governed by climate. Higher levels of carbonate deposition occur when the climate is warmer , as a result the levels of deposition can be clearly related to climate changes. The method which was used in this study involved the plotting of the ages and confidence intervals of the dated alluvial units for each of the catchment areas that were used in the study, in a latitudinal sequence from the north to the south. Based upon this, the oxygen isotope stages are measured, specifically 5 and 6. The number of alluvial units and the levels of precision to which they are dated appear to decrease to a large extent as the OIS 5e/6 boundary is reached. The mid point levels of the alleviation phases provide an indication of the age of the basin; when this is compared to the moderate and high resolution continental and marine paleoclimatic records, certain critical relationships emerged. The dating of the alluvial basins was possible based on the oxygen isotope stage at which the basin existed. The two methods mostly used in this study were luminescence dating and uranium series dating. Through dating of the alluvial and sediment deposits and arriving at an estimation of the variations in deposits at various levels, it was possible to determine the differences in the alluvial deposits at various stages in time, thereby providing an indication of how climatic changes have affected the Mediterranean basins at each of the eleven catchment areas that were identified in the study. Based upon the observations from the study and the use of both methods, the authors of this study have concluded that robust correlations can be made using these two methods between the late and middle Pleistocene terraces. The findings from the study suggest that some areas of the Mediterranean region have experienced considerable changes with major river aggradations occurring at certain ka levels, suggesting that climate changes have produced significant landscape changes. One of the important conclusions in this study was that during the late Pleistocene period, the river alleviations occurred during cool. Dry phases when steppes grasslands replaced the existing forests and the authors conclude that there may have been at least 13 alluviation episodes occurring in the Mediterranean region, which in turn have produced significant changes in the landscape. The oxygen Isotope stage 4 appears to have been a period of intense cold and aridity, and the climate fluctuations were more frequent and more intense during certain ka periods. The authors have also provided inferences on the reasons why certain changes to the river basins occurred as a result of aggradations. Since catchment areas in the northern Mediterranean regions lay close to the forests/steppes, a relatively small change in climate would have produced large effects on the slope vegetation, stability and run off response. In the high relief steep land Mediterranean catchments, the availability of good slope land coupling has produced a primarily coarse bed for alluvial deposits in this area and deposits demonstrate a high level of receptivity to climate changes. As opposed to this, lowland river systems in northern and western Europe demonstrate significant constraints in sediment availability and the lower levels of coupling between the hillsides and channels make them less responsive to climate changes. Another visibly notable effect of climate changes in the Mediterranean region has been the creation of a high resolution record of river response to climate changes in the late Pleistocene period through the formation of valley incisions and well developed river terraces. On an overall basis therefore, the authors have concluded that the major reasons for the changes in the Mediterranean river basin in terms of hydrology and vegetation over the last 200 ka appear to be the primary control of large sedimentation activity that has occurred between the time periods of 103 and 104 years. Although some of the changes have been caused by other aspects such as tectonic changes or geological structures, these aspects have only caused the river regions to become very sensitive to climate changes and this has in some aspects, also produced a damaging effect upon the environment and the vegetation of the region. The study has also provided suggestions and priorities for future research. The first priority which is identified is the need to further extend the scope and geographical coverage of the investigations into fluvial activity to also include areas in Italy, Turkey and North Africa. The second priority which has been identified is the need to carry out multiple dating assays of individual aggradation events that occur. The authors also point out that most studies have not investigated the timing of the deposits of sediments from hill slopes and tributary streams which have arisen as a result of rapid climate changes. Such information is however vital in the context of examining climate changes in the region. While the study has been useful in contributing an overall picture of the fluvial sequences, nevertheless the challenges presented lie in the documenting of the geomorphic effects of individual periods of rapid climate changes. According to Wallinga (2002), the optically stimulated luminescence method of dating fluvial sequences allows for the establishment of chronological frameworks for fluvial deposits and offers a significant advantage in this particular aspect. The OSL signal is comprised of quartz and feldspar minerals, which is reset when exposure to light occurs. The result of such exposure to light is a build up of the minerals as a result of ionizing radiation and the minerals are buried in the deposits. The advantage of this using this method is therefore that the fact that chronological frameworks can be established. The method is also associated with the disadvantage that when the levels of light available are inadequate, then the OSL signal may not set completely, which in turn would produce over estimations in the age of the deposits, especially in the case of samples which are not very old. Although methods are available to detect such incomplete resetting or poor bleaching, the relative effectiveness of such detection would largely depend upon the kind of materials used. Quartz is the most effective, because its OSL signal can be reset fairly easily. Applying this in the context of the Macklin et al (2002) study, it may be noted that the establishment of the chronology of river alluviation that has been derived in the study could be a fairly accurate assessment because the primary advantage of the optically stimulated method is to establish on a fairly accurate basis, the chronological frameworks for fluvial deposits. The authors have used the oxygen isotope method of analysis and this would appear to be a fairly accurate method of determining alluviation and deposition of sediment. As Fuller et al (1998) have also discovered in their study, the data gleaned from the use of luminescence dating techniques was found to be very accurate in a study of climate related environmental changes in north east Spain. This study found that river aggradational episodes occurred when stadial or neoglacial events occurred, while phases of river inclusion occurred during the interstitial or glacial periods. This corroborates the finding in the Macklin et al (2002) study which shows that coarse river beds have been produced as a result of the terrain and the nature of the activity that has taken place, i.e, coarse river beds have been formulated during a period of glacial activity, where the run off from the slopes has contributed to a change in the texture of the alluviation deposits and made them more coarse. As opposed to this, the lower lying areas have produced fewer variations in sediment deposition, hence producing aggradational patterns, while the lower lying areas tend to demonstrate more inclusive patterns of sedimentation. On the basis of the data gathered in the Macklin et al (2002) study, it may be noted that a wide range of latitudes have been taken into account, thereby accounting for river activity in almost the entire Mediterranean area, thus providing a sound basis to arrive at conclusions about climate change. One of the deficiencies identified in the study is the problems of effective correlation in the data that was obtained ; some of the alluviation events were found to be less coordinated and geomorphically effective and also less geographically widespread. This also produced changes in the nature of the sediment and alluvial content of the river basins. As detailed earlier, the OSL signals also provide clear indications of the manner and the extent to which climate changes have impacted upon the river basins, because those river basins which demonstrate a relatively low level of changes in terms of the nature of the sedimentary deposits and the fluvial content may not have been affected to such a high degree by the climactic changes as those which show more variation in terms of the texture and levels of sediment. The method which has been used by Macklin et al(2002) was based on correlations derived from the plotting of the ages and confidence intervals of the alluvial units. This method works especially well in this study, because it clearly illustrates those specific areas and/or river basins where there is incision and where there is aggradation. Moreover, it also enables the clear demarcation of those river basins where river terraces are present and where U/Th river terraces are present, or where U/Th fan type terraces are present. Through the plotting of the ages and confidence intervals of the alluvial units, it is also possible to visually demonstrate the not only the precise areas where the different kinds of alluvial basins exist, but also plot the time fframes during the respective Pleistocene periods when the specific kinds of river basins have developed, on the basis of the patterns of deposition that have taken place. Conclusions: On the basis of the above, it may be seen that the Macklin et al (2002) study has primarily focused on the method of optically stimulated luminescence, and the research tool/method which has been used is oxygen isotopes. This ensures that the results which have been derived in this study are likely to be fairly accurate. As may also be noted from the study, the scope of sedimentation and deposit activity which has been considered also includes the assessment of specific areas and time frames within which specific deposition patterns have yielded different kinds of alluvial activities. Notably, it may be seen that the review has used the method of plotting of confidence intervals and ages of the alluvial units, which enables a graphical repr4esaentation of alluvial depositional patterns, not only on the basis of the area where specific patterns occur but also the specific time frames within which specific kinds of depositional activities have taken place. It provides a visual representation of the fluvial activities which have occurred within the Mediterranean basin over a time frame of 200,000 years. Such plotting provides a long term view of the changes which have occurred due to climate and also the kind of impact it has had on the physical structure of the river basins and the alluvial environment. The efficacy of the methods used in the study also appear to be sound, because the use of the optical luminescence method has been shown to be one that enables a good chronological assessment to be made about fluvial patterns. Since primary reliance has been placed upon this method, it enables a good reconstruction of the chronology of events occurring as a result of the fluvial activity and deposition that have occurred in the river basins. The other method which has been used in the study, i.e., the uranium series dating method is also a useful one because it produces a clear indication of the extent to which changes have occurred due to climate, Therefore, when used in combination, the two methods have shown not only the extent of change which has taken place but also the degree of change in the alluvial deposit patterns, which enables excellent inferences to be drawn about how climate has impacted upon the development of different kinds of river basins and different levels of sedimentation in the river basins. On an overall basis, it may be concluded that the Macklin paper offers an excellent assessment of the impact of climate changes on fluvial alluviation in the river basins. The use of the optical luminescence is particularly relevant in this study because it allows for an accurate assessment of the chronological order of occurrence of events. The disadvantage which has been identified with the use of the optical luminescence method is that sometimes incomplete absorption of minerals might occurred due to inadequate levels of sunlight being available. This is not however a problem in the Mediterranean region, hence from that perspective, the disadvantages of the optical luminescence method are addressed or negligent, thereby making the advantages much more significant and allowing for the optimum use of this particular method in this context. The authors have also provided a good overview of the currently existing and available literature on the subject of fluvial depositation. The review has highlighted the findings of other authors on the subject of alluvial deposits and the impact of other studies on optical luminescence and the uranium dating methods. It has identified the weaknesses and strong points of some of these studies. Hence on an overall basis, it may be stated that Macklin’s study has provided a comprehensive review of fluvial activity in the river basins in the Mediterranean over a 20 ka year period. References: Fuller, Ian C, Macklin, Mark G, Lewin, John, Passmore, David G and Wintle, Ann G, 1998. “River response to high frequency climate oscillations in southern Europe over the past 200 k.y”, Journal of Sedimentary research, 26(3): 275-278. Macklin, M.G, I.C. Fuller, Lewin, G.S. Maas, D.G. Passmore, Rose, J.C. Woodward, S. Black, R.H.B. Hamlin and J.S. Rowan (2002). “Correlation of fluvial sequences in the Mediterranean basin over the last 20 ka and their relationship to climate change”, Quaternary Science Reviews, 21:1633-1641. Wallinga, Jacob, 2002. “Optically stimulated luminescence dating of fluvial deposits: a review”, Boreas, 31(4): 303-322. Read More