Gravity core shortening and pore water chemical gradients

Abstract

Profiles of pore water alkalinity and dissolved Fe, Mn, and PO 4 −3 were obtained for virtually undisturbed box cores and for gravity cores taken at the same stations. Linear alkalinity gradients in the box cores were preserved in the gravity cores, but the steepened gradients in the gravity cores indicate significant shortening. The ratio of the alkalinity gradients provides a measure of the degree of shortening. The relative shortening could also be clearly seen in the Mn, Fe, and PO 4 −3 profiles. We observed virtually no compaction and a linear shortening. The results agree with the mechanism proposed in early studies on gravity corer behaviour, namely the stretching of the sediment in advance of the corer and the recovery of a thinner section of sediment than the in situ interval. We observed no noticeable differences in pore water chemistry between box and gravity core samples, only the increased gradients due to thinning. The shortening (or thinning) factor at individual stations varied between 1.4 and 3.4, however, and for precise work it is suggested that a concurrent box core or other independent measures of the in situ gradients be obtained to correct for the true shortening.