RFID en el Sector Automotriz

As a result of the difficulty in obtaining a sufficient number of radiocarbon ages for a high resolution study of core UACT6, it was decided to construct a core chronology on the basis of lithostratigraphic correlation with UACT4, the dated core from the TIGGER Ila project. The strong correlations between dry weight and LOI measurements of UACT3 and UACT4 have previously been established using a sequence slotting algorithm (Figure 3.2). Although the correlation between UACT6 and UACT4 was not as well-defined, the main features were nonetheless apparent.

The most likely correlation between UACT6 and the two longer cores is given in Figure 3.7. The correlation is made visually, as it is felt that use of a numerical sequence slotting programme is only justified where a far better correlation is apparent between cores. The top half of UACT6 (0-c. 25 cm depth) appears to have a slightly higher overall accumulation rate than UACT4, which itself has a higher accumulation rate than UACT3. From 25 cm depth to the core base the overall accumulation rate is lower than for the corresponding section of UACT4, and slightly lower than for UACT3. The base of UACT6 at 46.2 cm is thought to correspond to a depth of 48 cm in UACT3, and 53 cm in UACT4.

Differences between the cores exist. For example, the section from 3-15 cm depth in UACT6 exhibits low variability in LOI values. This is not seen in either UACT3 or UACT4. Such features may be due to variations in accumulation rates within and between cores (see below). Disturbance during coring cannot be ruled out, but is thought unlikely due to the lack of smearing in the signal, as shown by the sharp features apparent in the UACT6 LOI profile, and the lack of visible disturbance at the mud-water interface at the time of coring.

UACT6 exhibits larger amplitude variations than UACT3 and UACT4, and more overall noise between samples. Maximum and minimum values of LOI in UACT6 are

UACT3 LOI/% dry wt. 10 15 20 25 0 UACT4 LOI/% dry wt. 10 15 20 25 30 0 10 UACT6 LOI/% dry wt. 15 20 25 30 35 I4 1 5 ± 6 0 9.M) ± 50 1480 ± 45 1745 ± 45 2200 ± 85 2220 ± 55 Depth cm 2495 ± 55 3105 ± 5 0 - 3350 ± 5 5 - 3395 ± 55 - 3495 ± 55 - 4325 ± 55 - 4170 ± 5 5 - 4515 ± 6 5 - 100 110

Figure 3.7 Correlation between LOI profiles for cores UACT3, UACT4 and UACT6. Uncalibrated radiocarbon ages for UACT4 are shown.

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larger or smaller respectively than for UACT3 and UACT4. In particular, the topmost samples of UACT6 show a rise in LOI to >30% at the surface. A corresponding, but smaller magnitude, rise is seen in UACT4, and barely any increase in UACT3. This is possibly due to differences in subsampling of the cores by different analysts. Topmost samples of UACT6 have a high water content, and appear to consist principally of organic material in suspension. This may reflect a delay in the mineralisation of organic matter at the mud-water interface, or the presence of rapidly-degradable organic components such as polysaccharides. The conditions at the mud-water interface of cores UACT3 and UACT4 during subsampling are not known.

Although core UACT6 was analysed by a different analyst from cores UACT3 and UACT4, the possibility of differences in LOI due to the use of different methods is considered slight. All cores were analysed in the same laboratory, using the same equipment and following a standard procedure (Bengtsson and Enell, 1986). LOI is sometimes thought to be unreliable in sediments with a high clay content where water may be chemically bound to the clay as iron, aluminium or manganese oxides (Mackereth, 1966; Hâkanson and Jansson, 1983; Sutherland, 1998). This water is not removed during drying at 105°C, but is lost by dehydration at 550°C, thus contributing to the ‘organic’ weight. However, this problem is thought to be minimal at Lochan Uaine as the sediment has a low clay content, and is dominated by silt-sized siliceous diatom remains. The LOI results are corroborated by measurements of TOC which show a highly significant correlation with LOI, with an R^-value of 0.865 (Chapter 4). Furthermore, differences are seen between the LOI profiles of UACT3 and UACT4. These profiles were both measured by the same analyst, suggesting that differences between core profiles are real and not due to inconsistencies in the methods used by different analysts.

Average sediment accumulation rates differ between the three cores, as noted previously with respect to the correlation of the base of UACT6 with the corresponding depths in UACT3 and UACT4. These were given as 46.2 cm, 48 cm and 53 cm respectively. In addition, accumulation rates are not constant within any one single core. This is demonstrated by the alternate bunching and spreading of the

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lines depicting depth correlations between UACT3 and UACT4 (Figure 3.2). These are small variations, generally within the range of at most a couple of centimetres. Nonetheless, the varying accumulation rates have imphcations for radiocarbon dating, in particular the use of a linear regression to construct the main depth-age model. These problems are discussed in more detail below (Section 3.5.1).

Assuming that the correlation between cores UACT4 and UACT6 is accurate, it should then be possible to use the dating of UACT4 as a means to date UACT6. It is first necessary to evaluate the accuracy and precision of the dating of UACT4, with the aim of establishing a calendiical depth-age model. The use of calendar dates, rather than uncalibrated radiocarbon ages, is essential to allow comparisons to be made between the sediment record and other non-radiometric ally dated sequences, such as ice core, tree ring and instrumental records.