High-frequency paleoclimate signals from Foulden Maar, Waipiata Volcanic Field, southern New Zealand: An Early Miocene varved lacustrine diatomite deposit

A freshwater diatomite deposit near Dunedin, South Island, New Zealand provides an exceptional archive of mid-latitude, seasonal climate variation during latest Oligocene-early Early Miocene time. The diatomite accumulated in a small ~ 1.5 km diameter maar-floored lake that formed during basaltic volcanism. Except for exposures of basanite dated at 23.2 Ma by 40Ar/39Ar, and remnants of a Paleogene sandstone and conglomerate cover, the diatomite body is surrounded by Otago Schist. Geophysical profiles indicate that the total diatomaceous sediment thickness may exceed 100 m. Two depositional facies are described from 15.5 m of weakly-consolidated fresh diatomite accessible in two pits. A thinly laminated facies, comprising 60% of the section, consists of dark brown and white couplets of average thickness 0.5 mm. Both brown and white laminae are composed essentially of frustules of a single pennate diatom, along with 1-2% of siliceous sponge spicules. Dark laminae also contain abundant 5-7.5 µm diameter siliceous chrysophycean stomatocysts and organic matter. Decalcified complete skeletons of Galaxias, a southern hemisphere group of freshwater or diadromous fish, are found throughout. Diverse well-preserved leaves, some with attached scale insects, are dominated by Lauraceae. Rare flowers are also present. Trace fossils preserved in the laminated comprise 1-1.5 mm wide pale diatomaceous strings of probable faecal origin, and isolated 20-40 mm diameter sand lenses interpreted as faecal remains of swimming waterfowl. The second diatomite facies group comprises dark brown speckled beds, interpreted as sediment gravity flows. Up to 14 cm thick and interspersed throughout the study section, they incorporate abundant laminated diatomite flecks and leaves, and minor woody plant matter and terrigenous silt. Almost all are capped by 1-8 mm of white diatomite that is interpreted as post-flow fall-out of resuspended diatom frustules. Associated breccia and swirly beds up to 8 cm thick are composed of variably contorted clasts of laminated diatomite in a speckled matrix. Nodules, spheroids, and films of authigenic framboidal pyrite are commonly associated with plant and fish remains. As in modern lakes, white laminae likely formed from diatom blooms when light flux, temperature, or nutrients were optimal during spring-early summer; dark laminae accumulating during autumn-winter months. Laterally continuous pinstripe lamination, absence of bioturbation, and excellent fish and plant preservation indicate that the profundal lake and floor were anoxic. Periodic variations in couplet thickness of ~ 3-10 years compare with Quaternary records of El Niño-Southern Oscillation variability and indicate that the early Early Miocene low altitude New Zealand climate was seasonal and strongly ocean-influenced, as it is today.