Be-10/Be-9 dating of ferro-manganese nodules from the NE Pacific Ocean

Be-10 and Be-9 concentrations in two ferro-manganese nodules from the Clari on-Clipperton fracture zone in the northeastern equatorial Pacific Ocean ha ve been measured at the Rafter Accelerator Mass Spectrometry facility at GN S, New Zealand. The Be-10/Be-9 ratios within the nodules decrease from the rim to the core which. assuming a constant uptake of Be-10 and Be-9 during growth, can be related directly to the nodule age and growth rate. One of t he nodules (A03-3) has been subdivided into two textural zones, both of whi ch are indicative of early diagenetic growth. The inner zone (23.8-46.3 mm from the rim) yields a growth rate of 28.3 mm My(-1), and the outer zone (0 .0-23.8 mm) a growth rate of 22.5 mm My(-1). There is no evidence of any ti me break between the zones, indicating that growth was continuous throughou t the nodule's history, beginning at c. 1.9 Ma. The other nodule (B02-1) ha s an inner textural zone (5.5-11.5 mm) which yields a relatively slow growt h rate of 4.4 mm My(-1), indicating hydrogenetic growth from c. 3.3 Ma. Bet ween the inner and the outer textural zones is a thin lamination, represent ing an age gap of 1.45 My, when growth was very slow and some re-crystalliz ation took place. The outer textural zone (0.0-5.4 mm) yields a faster grow th rate of 9.9 mm My(-1), indicating diagenetic growth. Be-10 dating of Fe- Mn nodules, while now considered to be relatively robust, can be applied in several different ways with varying degrees of uncertainty. Whatever model is used, it must be considered with regard to observable textural variatio ns within the nodules. The precise nature of the growing environment and th e process of element accumulation remain equivocal, however, but are relate d closely to the role of associated sediments and the influence of deep oce an currents.