Large kelp-rafted rocks as potential dropstones in the Southern Ocean

Abstract

Cyclonic winds in April 2017 forced oceanic kelp rafts onshore in southeastern New Zealand, providing a ‘snapshot’ of ongoing kelp-rock transport. About 10% of observed southern bull kelp (Durvillaea antarctica) holdfasts contain substantial remnants of their original rocky substrates. The largest rock weighs ~ 7 kg, and was derived from the Mesozoic sedimentary Murihiku Terrane of southern New Zealand, > 100 km from the beach where it was deposited. Several other rafted rocks were derived from the same general source area, including a conglomerate clast with 10 cm igneous cobbles. Genetic sequencing of the kelp specimens confirmed their southern New Zealand derivation, which is distinct from sub-Antarctic or northern New Zealand clades. The floating rocks had been colonised by goose barnacles (Lepas australis), the largest of which were approximately 5 mm long, implying that the rocks had been floating for approximately 16 days before being blown ashore. The rocks may have travelled up to 200 km in that time in a northeasterly directed ocean current that flows at 10–15 km/day. If these rock-bearing kelp rafts had not been blown ashore in the cyclonic weather event, they would likely have continued to travel northeast in the Subtropical Front zone where they would likely have been deposited as dropstones. Abundant dropstones noted on the nearby Chatham Rise and Campbell Plateau, have previously been assumed to have been ice-rafted. A component of these dropstones could have been derived by kelp rafting, especially on the Chatham Rise, where the Subtropical Front is deflected east. A thick weathering rind, and remnant organic veneer, if present, along with clast provenance, may be the best distinguishing features of kelp-rafted dropstones compared to ice-rafted dropstones.