Temporal duration and preservation mechanism of the Middle Triassic Luoping biota from South China constrained by geochronology and cyclostratigraphy

The Middle Triassic Luoping biota represents a full recovery of marine ecosystem after the Permian-Triassic mass extinction, yet the temporal duration and exceptional preservation mechanism of the Luoping biota remain uncertain. Here, zircon geochronology and cyclostratigraphy analysis are carried out on an Anisian (Middle Triassic) Guanling Formation succession yielding the Luoping biota at the Xiangdongpo section in Yunnan Province, South China. The sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of a tuff bed (bed 15) in the strata confirmed an age of 244.5 +/- 2.2 Ma for the Luoping biota. Spectral analyses on the elemental proxies for terrestrial flux (K, Ti, Si, Al, and K/Ti) reveal significant sedimentary cycles. The cyclo-stratigraphy analysis shows persistent cycles of ca. 4.4-5.1 m, 1.7-2.0 m, 0.9-1.2 m,-0.5 m, 0.33-0.37 m, and -0.23 m cycles, which may correspond to the orbitally forced sedimentary cycles caused by short-eccentricity (-100 kyr), obliquity (-32.9 kyr), precession (-20 kyr), and sub-Milankovitch cycles (-10 kyr,-8.3 kyr, and -5 kyr). Based on these results, we estimate the duration of the Luoping biota to be-340 +/- 71 kyr. The intensified monsoonal climate during eccentricity maxima modulated large amplitude of variations of-5 kyr quarter precession,-10 kyr half precession, and -20 kyr precession cycles, led to high terrigenous discharge and strengthened upwelling, which fueled the primary productivity in surface water due to high nutrient input into the Luoping area. We propose that this process enhanced oxygen consumption in the water column, trig-gered anoxia of bottom water, and promoted the exceptional preservation of the Luoping biota.