Solar cycles recorded in Carboniferous glacimarine rhythmites (Western Argentina): relationships between climate and sedimentary environment

A Carboniferous glacial episode has been widely known in the western part of Argentina since ca. 1910. In some sections it is associated with rhythmic sedimentation. One of those rhythmite successions is interpreted as composed by varves (annually deposited sedimentary cycles) accumulated in a glacimarine environment. It is further analyzed as a time series searching for cyclic signals. Several localities show varve-like successions, but only a few of them are not disturbed by reworking of bottom currents. The Rı́o Francia section (San Juan, Argentina) is well-suited for varve-counting as no current-produced structures are recognized through the rhythmite interval, and outcrops are good enough for counting directly in the field. Five varve-types which show differences mainly in thickness, grain size and dropstone frequency are recognized. Internally, they are mostly symmetric, distinguishing them from traditional normally graded varves, a feature that has also been observed in Holocene glacimarine varves. The different varve types indicate varying positions with respect to the ice front. Because of disturbances or lack of well defined textural or colour contrasts, only the three varve types with intermediate thicknesses were counted. Three units including 1028, 690 and 465 varves were measured and then analyzed as time series in order to detect cyclic signals. Some differences in the quality of the cyclic signals can be seen in spectral analyses, depending on the main type of the varve that composes the unit. However, a peak around 12 yrs can be seen in most of the spectra, and is interpreted as the sunspot-cycle record. A secondary peak near 24–26 yr also may be related to this phenomenon (Hale cycle). Four major cycles, recognized in the field but not in the spectra, that represent major glacial advances and retreats, can also be related to a solar periodicity of 2500–3000 yr. The recognition of sunspot cycles in these Carboniferous varves is not unusual as they are found in other ancient varve successions. Random processes, typified by the distribution of dropstones, played an important role in disturbing the climatic signal in this sedimentary environment. The random process of sediment transport and deposition by ice rafting disturbs the cyclic climatic signal mainly in proximal varve-successions. Increased water motion and distance to the source of sediment toward the open sea tend to produce more homogeneous very fine-grained distal varves, eventually causing the loss of varve-like appearance. The remaining varves located at intermediate positions with respect to the glacio-related sediment source seem to be the best suited for recording climatic cyclicity, as shown by spectral analyses.