9:00 AM - 9:15 AM
[MIS10-01] Solar influence of decadal- to millennial-scale climate variability in the mid-Cretaceous Supergreenhouse: Insights from a Mongolian lacustrine record
Keywords:decadal-scale, millennial-scale, Climate variability , Lacustrine varve, Cretaceous, Solar activity
Here, we present evidence of decadal- to millennial-scale climate variability during the mid-Cretaceous (Early Aptian) from a Mongolian lacustrine record (Shinekhudag Formation). Depositional age of the Shinekhudag Formation is defined as between 123.8–118.5 Ma based on the radiometric age constraints (Hasegawa et al., 2018). Strata of the formation is composed of rhythmically alternating beds of dark grey shale and light greydolomite, interpreted to reflect orbital-scale precipitation changes. Based on seasonally-resolving analyses of the varve record present in the shale beds by using a fluorescent microscope, we reconstructed changes in algal organic matter flux, most probably reflecting changes in lake surface productivity controlled by strength of summer insolation, for a 5.5 cm thick interval (corresponding to ca. 1090 yr duration). The algal organic matter flux proxy shows periodicities of ca. 3–5, 11, 35–40, 90–120, 220, and 360–400 years. The obtained cyclicities seem to correspond to the well-documented solar activity cycles (11 year Schwabe cycle, 88–105 year Gleissberg cycle, and ~210 year de Vries cycle). Particularly, changes in algal productivity appear to reflect a pronounced 11 year Schwabe cycle, which is not so obvious in Holocene record (e.g., Czymik et al., 2016; Novello et al., 2016).
We further performed decadally-resolving elemental composition analyses for a 20 m thick core interval (corresponding to ca. 200 ky duration) by using a μXRF core scanner (COX, ITRAX), in order to reconstruct centennial- to millennial-scale climatic variability in the mid-Cretaceous. The precipitation proxy (Ca/Ti) record shows periodicities of ca. 400–500, 1000, 1400–1450, 2000–2300, and 3500–4000 years. The 1000 and 2300 years’ cyclicities are corresponding to Eddy and Hallstatt cycles of invoked long-term solar activity change. Thus, our finding suggests that a solar influence of decadal- to millennial-scale climatic oscillations existed during the mid-Cretaceous “Supergreenhouse” period. In addition, the variation pattern of the 1400–1450 years’ cycle observed in the Mongolian lake record appears to be similar to the DOC of the last glacial. Many studies (e.g., Barker et al., 2015; Buizert & Schmittner, 2015) excluded a solar forcing origin of ca. 1500 year’ climate variability of DOC and interpreted the DOC to be triggered by internal variability of either polar iceberg calving or oceanic circulation changes in the North Atlantic and Arctic oceans. However, lake records of both the Miocene (Kerns et al., 2012) and the mid-Cretaceous (this study) revealed that existence of similar periodicities of climate oscillations, although polar-ice volume and land-ocean distribution settings in both periods are largely different compared with the last glacial. Therefore, our finding supports the idea of persistent external pacing of 1400–1500 year’s climate variability (Bond et al., 2001; Braun et al., 2005). As noted by Muscheler (2012), although 1500 year cyclicity is not observed in spectral characteristics of solar variation records in the Holocene, it may be still premature to rule out solar forcing altogether. Our findings further suggest the existence of another climate stability mode which strengthened millennial-scale climate oscillations in the mid-Cretaceous “Supergreenhouse” world, not only in the glacial mode (Barker et al., 2011).