Paleomagnetic observations from lake sediments on Samosir Island, Toba caldera, Indonesia, and its late Pleistocene resurgence

Approximately 74 ka, Toba caldera in Sumatra, Indonesia, erupted in one of the most catastrophic supereruptions in Earth's history. Resurgent uplift of the caldera floor raised Samosir Island 700 m above Lake Toba, exposing valuable lake sediments. To constrain sediment chronology, we collected 173 discrete paleomagnetic 8 cm3 cubes and 15 radiocarbon samples from six sections across the island. Bulk organic 14C ages provide an initial chronostratigraphic framework ranging from ~12 to 46 ka. Natural and laboratory magnetizations were studied using alternating field demagnetization. A generally well-defined primary magnetization is isolated using principal component analysis. Comparison of inclination, and to a lesser degree declination, across independently dated sections suggests paleomagnetic secular variation (PSV) is recorded. Average inclination of −6° is more negative than a geocentric axial dipole would predict, but consistent with an eastward extension of the negative inclination anomaly observed in the western equatorial Pacific. The 14C- and PSV-derived age model constrains resurgent uplift, confirming faster uplift rates to the east and slower rates to the west, while suggesting that fault blocks moved differentially from each other within a generally trapdoor-type configuration.