El proceso, la prueba y la prueba de oficio en la óptica del dispositivismo

The weight of a non-porous, empty crucible (a blank) weighing 1.0 g was programmed into the carbon and sulphur determinator machine and the crucible placed onto a tray using tongs, this procedure was carried out three times. The machine calibrated a value from the three blanks which is subtracted from the values of subsequent samples. A non-porous crucible was placed on the scales which was then zeroed. A steel ring which acts as a standard was then placed into the crucible and the weight automatically registered by the machine. The crucible was then placed onto a tray using tongs. Four standards were used for each run with a known carbon and sulphur content of 0.35% and 0.0270%, respectively. Each sample of mudstone was sieved through an 80 |im sieve. Between 0.7 and 0.9 g of the residual powder of each sample was placed in a porous ceramic crucible and the weight automatically registered by the machine. The crucible was then placed onto a tray using tongs. 30 samples were analysed per run. Each crucible was filled with 10% hydrochloric acid (HCl) to remove the carbonate content. When the reaction with the initial amount of HCl was completed the solution was pumped out of each crucible and more HCl

added. Once the reaction with each sample was complete the solution was again pumped out of the crucible. The HCl was flushed from the sample by adding de­ ionised water which was left to drain from the crucible. This procedure was repeated until all reactions were complete. The tray of 30 samples was placed in an oven at 45°C overnight to remove all moisture.

After preparation of the samples, the machine was checked for any leaks in the system. In order to zero the machine an empty crucible was placed in the furnace chamber of the machine where it was inductively heated to >1000°C in a stream of pure oxygen. This was done for all three of the empty crucibles. A steel ring with a known carbon composition (0.35% carbon) was placed into a crucible. The steel ring is a standard from which the carbon content of the sample is determined. Iron chips were poured into the crucible containing the steel ring standard to act as an accelerator focusing the radiation into the centre of the crucible. Four crucibles each containing a steel ring standard were treated in this way. Iron chips were added to all the crucibles containing the powdered samples. Each of the 4 crucibles containing the steel rings were placed in the furnace chamber of the machine and inductively heated to >1000°C and the carbon and sulphur content calculated. Once the machine had measured all 4 of the standards the average was used to calibrate the machine for determining the carbon content of each sample. Once the machine was calibrated, each of the crucibles containing the powdered sample was placed in the machine and heated to >1000°C and the carbon content measured.

Inductively heating the standards and samples to >1000°C ensures the complete combustion of carbon and organic compounds to CO2. The machine then determines

the volume of CO2 flowing past the detectors and integrated this value with that from

the standards, which was then compared with the weight of the sample to give the % carbon value. The running time for each sample was approximately 1.5 minutes and a total of 30 samples per run were analysed.

Chapter 5

Geochemical Results

5.1 5 1 * 0 and results

The isotopic results of the planktonic {Globorotalia inflata) and benthonic

{Bolivina robusta and Bolivinita quadrilata) foraminfera versus depth are presented in Fig. 5.1. The results from the Globorotalia inflata are the most useful in this study because they provide a continuous isotopic record throughout the section. Also, the benthonic foraminfera are infaunal, and therefore are regarded as recording the isotopic composition of interstitial water (pore water) rather than the composition of the ambient seawater.

The stable isotopic record of Globorotalia inflata shows the signal is heavier than the signal, a feature commonly seen with benthonic species (Fig. 5.1). The average depth of shell secretion of Globorotalia inflata is 400 m (Oba & Yasuda 1992), i.e. within the mesopelagic depth zone as defined by Edwards (1979) for water depths from 300 - 1200 m. The present day mean water temperature in this depth range offshore from Boso Peninsula is 12.5°C in summer and 14°C in winter {ibid.).

In the upper 100 m of the water column seasonal temperature fluctuations range from 3 - 10°C compared to 1.5°C fluctuation at 400 m (Oba & Yasuda 1992). As variations in water temperature are less below 1 0 0 m, there is a corresponding reduction in the

range of Ô^^O values, therefore any significant change in the Ô^^O signature of

Globorotalia inflata is interpreted as a function of glacio-eustasy.

5.1.1 0^*0 of Globorotalia inflata

The 6^^0 curve of the planktonic species Globorotalia inflata shows a high frequency, low amplitude variation ( < l % o ) in the lower 850 m of the 1500 m thick

%oPDB “/ooPDB 1500 t r o Globorotalia inflata Bolivinita quadrilata 1000 — 1000 LL Bolivina robusta a . 5 0 0 --- 500 Fig. 5.1

section and low frequency, high amplitude variation (> l% o) in the upper 650 m of the 1500 m thick section. The mean signal increases from 0.9% o below 850 m to 1.25°/oo above 850 m, an increase of 0.35°/oo- (Fig. 5.2). The transition between the two different modes of isotopic variation occurs 350 m below the Bruhnes-Matuyama boundary (located in the middle Kazusa Group by Niitsuma 1976).