SEDIMENTATION IN THE DEEP SEA 639 



more complicated, possibly due to radium leaving its mother element 

 ionium through a diffusion process. Anyhow approximative age-deter- 

 minations, based on the radium content near the sediment surface have 

 given rates of sedimentation as low as 1 mm. or even less in 1,000 years 

 for the central Pacific Ocean. 



A perfected method for uranium determinations, elaborated by 

 Hecht in Vienna in collaboration with Kroll, has shown that the high 

 radium concentration found in the surface of Pacific Red Clay, up to 

 50 units of the 12th decimal place, cannot be explained as due to ura- 

 nium-supported radium. The uranium content is rarely more than a 

 few units of the 6th decimal place corresponding to only 1 iniit of the 

 12tli decimal place for radium. 



Thanks to the recent work by E. Picciotto in Brussels, adopting 

 the photographic method, it has for the first time become possible to 

 measure the concentration of ionium and of its isotope thorium in deep- 

 sea deposits, a technique which may open a more certain ^vay towards 

 radioactive age-determinations. 



Summing up the results found from these different methods, we 

 may say that the sedimentation rate in the equatorial Pacific Ocean 

 is of the order 1 to a few mms in 1,000 years for Red Clay, and twice 

 to four or even five times higher for calcareous ooze. One of our longest 

 Red Clay cores from the central Pacific Ocean of 15 meters should, 

 therefore, with its lowest strata carry us back 15 to 20 million years in 

 time. 



However, studies of the East Pacific cores by Arrhenius and of the 

 Radiolarians in the deep-sea sediments by Riedel have proved that there 

 are cases when the rate of accumulation of sediment assumes iiegative 

 values, owing to the upper layers having been carried away by eroding 

 bottom currents laying bare older sediments.* This also may lead to 

 radiolarians of Tertiary age becoming mixed in the same layer with 

 more recent ones. 



The most surprising results from the soundings of the sediment 

 thickness made by Weibull's reflexion method, was the small thickness 

 of the sediment layer found in the Pacific and the Indian Oceans, rarely 

 over a few hundred metres, whereas in the Atlantic Ocean between Ma- 

 deira and the Midatlantic Ridge WeibuU found a maximum thick- 

 ness of over 3,400 metres. The low values from the other oceans have 

 recently been confirmed through measurements by the refraction meth- 

 od, carried out from American and British expeditions. These surpris- 

 ingly low values are only a fraction of the theoretical thickness com- 

 puted by Kuenen and others. For this large discrepancy no satisfactory 

 explanation has so far been advanced. 



• Already reported from the NW Atlantic Ocean by M. Ewing & Co-workers. 



