Foraminiferal and molluscan evidence for the Holocene marine history of two breached maar lakes, Auckland, New Zealand

Abstract

Drillhole records of fossil Foraminifera and Mollusca, together with sparse tephra age control, document similar Holocene marine histories of two of Auckland's breached maars—Pukaki Lagoon, Manukau Harbour, and Onepoto Lagoon, Waitemata Harbour. Following eruption, both maars slowly accumulated carbonaceous mud in freshwater lakes, until they were breached by rising sea level in the early Holocene (c. 8100 cal. yr at Onepoto, c. 7600 cal. yr at Pukaki). Following breaching, both became saltwater tidal lagoons with silled, subtidal basins rapidly accumulating marine mud as the underlying sediment compacted. Onepoto Lagoon may have had deeper water than Pukaki, because it was colonised by a foraminiferal fauna (Bolivina, Bulimina, Buliminella, Spiroloxostoma) that prefers quiet, dysoxic bottom conditions. Both fossil groups identify where the lagoons shallowed from subtidal to low tidal depths. This occurs c. 15 m downhole (6900 cal. yr) in Pukaki and c. 9.5 m downhole in Onepoto, after sea‐level rise had levelled off at about its present height (7000 cal. yr). Marine mud sedimentation slowed in the intertidal, accumulating largely in response to 12 m and 5 m compaction of the maar fill, respectively. Subtidal and low tidal fringe foraminiferal faunas of both lagoons are characterised by Ammonia‐Haynesina associations, whereas intertidal faunas above mean low water are dominated (>90%) by Ammonia. Pukaki Lagoon foraminiferal faunas differ from Onepoto by their higher subtidal diversity of benthic foraminiferal tests and the presence of planktic tests in the subtidal section. These differences are inferred to relate to the significantly more exposed conditions outside the entrance to Manukau Harbour, where juvenile benthic tests were lifted into suspension and, together with the planktics, carried by the strong tidal currents up the harbour channels into Pukaki Lagoon. These introduced tests settled out of suspension in the quiet subtidal waters and accumulated in the sediment. Once Pukaki Lagoon had been filled with mud to intertidal depths, most introduced tests were apparently flushed away by the outgoing tides and did not accumulate. The presence in the Onepoto sequence (9.8–8.7 m) of the gastropods Micrelenchus huttonii and Notoacmea helmsi f. scapha indicate that Zostera seagrass once grew in the lagoon at around spring low tide level.