Significance of Short-Wavelength Magnetic Anomaly Low Along the East Pacific Rise Axis, 9°50'N

Magnetic anomaly variations near mid-ocean ridge spreading centers are sensitive to a variety of crustal accretionary processes as well as geomagnetic field variations when the crust forms. We collected near-bottom vector magnetic anomaly data during a series of 21 autonomous underwater vehicle Sentry dives near 9 degrees 50'N on the East Pacific Rise (EPR) covering similar to 26 km along-axis. These data document the 2-3 km wide axial anomaly high that is commonly observed at fast-spreading ridges but also reveal the presence of a superimposed similar to 800 m full wavelength anomaly low. The anomaly low is continuous for >= 13 km along axis and may extend over the entire survey region. A more detailed survey of hydrothermal vents near 9 degrees 50.3'N reveals similar to 100 m diameter magnetic lows, which are misaligned relative to active vents and therefore cannot explain the continuous axial low. The axial magnetization low persists in magnetic inversions with variable extrusive source thickness, indicating that to the extent to which layer 2A constitutes the sole magnetic source, variations in its thickness alone cannot account for the axial low. Lava accumulation models illustrate that high geomagnetic intensity over the past similar to 2.5 kyr, and decreasing intensity over the past similar to 900 years, are both consistent with the broad axial anomaly high and the superimposed shorter wavelength low. The continuity of this axial low, and similar features elsewhere on the EPR suggests, that either crustal accretionary processes responsible for this anomaly are common among fast-spread ridges, or that the observed magnetization low may partially reflect global geomagnetic intensity fluctuations. Plain Language Summary Near-bottom magnetic anomaly data provide valuable information on crustal accretion processes at mid-ocean ridges. Using autonomous underwater vehicle data, we analyze near-bottom magnetic anomalies at the EPR 9 degrees 50'N to study crustal accretion at mid-ocean ridges. We find a continuous axial anomaly low superimposed on the typical broad axial magnetic high, located along the spreading axis covering an along-axis distance of more than 10 km. This axial anomaly low has been observed at other fast-spreading mid-ocean ridges, but the cause of the anomaly low is not well understood. Using magnetic inversion results and numerical models, we consider three possible causes for the axial anomaly low: variations in the thickness of the pillow lavas that are typically modeled as being the primary contributor to the magnetic signal, variations in Earth's geomagnetic field intensity, and hydrothermal vents chemically altering the magnetic minerals in the seafloor basalts. While the global occurrence of the axial anomaly low at other ridges makes it likely that geomagnetic field variations contribute to the low, it is likely that the observed axial magnetic low is caused by a combination of these three factors.

Automated summary

By analyzing magnetic anomaly data near 9 degrees 50'N on the East Pacific Rise, researchers have discovered a continuous axial anomaly low, which is likely caused by variations in Earth's geomagnetic field intensity.