Stratigraphy of a Sediment Core Collected from the NE Equatorial Pacific Using Reversal Patterns of Geomagnetic Field and Be Isotope Ratio

Abstract

A 570 cm-long sediment core was retrieved at 9 o 57'N and 131 o 42'W in 5,080 m water depth from the northeast equatorial Pacific and its stratigraphy was established with 10 Be/ 9 Be and paleomagnetic measurements. Successive AF demagnetization reveals eight geomagnetic field reversals. In the reference geologic time scale, the eight reversal events correspond to an age of about 4.5 Ma. However, 10 Be/ 9 Bebased age yields 9.5 Ma at a depth of 372 cm. Such a large discrepancy in determined ages is attributed to an extremely low sedimentation rate, 0.4 mm/kyr on average, of the study core and resultant loss or smoothing of geomagnetic fields. The composite age model reveals a wide range in the sedimentation rate -varying from 0.1 to 2.4 mm/kyr. However, the sedimentation rate shows systematic variation depending on sedimentary facies (Unit II and III), which suggests that each lithologic unit has a unique provenance and transport mechanism. At depths of 110-80 cm with a sedimentation rate of about 0.1 mm/kyr, ancient geomagnetic field reversal events of at least a 1.8 Myr time span have not been recorded, which indicates the probable existence of a hiatus in the interval. Such a sedimentary hiatus is observed widely in the deepsea sediments of the NE equatorial Pacific.