NEW SHIPBOARD MAGNETIC AND GRAVITY DATA FROM THE CRETACEOUS ELLICE BASIN

R/V Kilo Moana expedition KM1609 to the Ellice Basin set out to test the hypothesis of the proposed Ontong Java-Manihiki-Hikurangi breakup and subsequent formation of the Ellice Basin during the Cretaceous Normal Superchron. In addition to swath mapping, dredging, and sub-bottom profiling, we deployed a Geometrics G-882 cesium-vapor marine magnetometer and a Bell Aerospace BGM-3 gravity meter. Because the magnetometer is of the flux-gate type, and because the study area is in close proximity to the magnetic equator, we had fortunately designed our survey lines to run parallel to fracture zone trends and perpendicular to the relict, short-segment spreading centers (i.e., ~112 deg and its reciprocal course). In the eastern mapping area, we needed to rotate the magnetometer's sensor to 45 degrees to optimize signal strength. Preliminary magnetic anomalies were calculated at sea by subtracting from each total field measurement a main field intensity estimate from the 2015 IGRF. Preliminary magnetic anomalies distribute about zero in unimodal, approximately normal fashion and range from -601.15 to 691.47 nT with mean, median and standard deviation of 14.45, 9.55, and 132.14 nT, respectively. Shipboard gravity measurements were subjected to a six minute Gaussian filter, corrected for vessel motion to remove Eötvös effects, and reduced to free-air anomalies by subtracting normal gravity. Free-air anomalies are also unimodally distributed about zero, ranging from -289.0 to 718.8 mGal with a mean, median, and standard deviation of -10.8, -13.1, and 26.5 mGal. This negative gravity shift is also apparent in satellite gravity. Magnetic and gravity data were merged with vessel navigation, center beam depths, and other ancillary data automatically every several minutes, to create a complete file in the standard MGD77T format suitable for archival at the National Center for Environmental Information. Once the archive file was created, subsequent steps including sanity checking, extraction, and visualization were easily accomplished using GMT and the mgd77 toolkit. By creating the archive file prior to scientific analysis, we ensure that it is vetted by the science party and that the same information also goes to the data repository. We will combine new and existing data and present preliminary interpretations.