Publisher: PANGAEA - Data Publisher for Earth & Environmental Science
Project: NSF | Management and Operations... (1326927), EC | EARTHSEQUENCING (617462), UKRI | Exploring the roles of oc... (NE/R012350/1)
The geologic time scale for the Cenozoic Era has been notably improved over the last decades by virtue of integrated stratigraphy, combining high-resolution astrochronologies, biostratigraphy and magnetostratigraphy with high-precision radioisotopic dates. However, the middle Eocene remains a weak link. The so-called "Eocene time scale gap" reflects the scarcity of suitable study sections with clear astronomically-forced variations in carbonate content, primarily because large parts of the oceans were starved of carbonate during the Eocene greenhouse. International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). The sequence consists of nannofossil chalk and exhibits rhythmic clay content variability. Here, we show that IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for our U1514 astrochronology. We present a 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity. We supplement the high-resolution XRF data with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM - ca. 56 Ma) into the middle Eocene (ca. 40 Ma). Our early Eocene astrochronology corroborates existing chronologies based on deep-sea sites and Italian land sections. For the middle Eocene, the sedimentological record at U1514 provides a single-site geochemical backbone and thus offers a further step towards a fully integrated Cenozoic geologic time scale at orbital resolution.
Coherent variation of CaCO3 burial is a feature of the Cenozoic eastern equatorial Pacific. Nevertheless, there has been a long-standing ambiguity whether changes in CaCO3 dissolution or changes in equatorial primary production might cause the variability. Since productivity and dissolution leave distinctive regional signals, a regional synthesis of data using updated age models and high-resolution stratigraphic correlation is an important constraint to distinguish between dissolution and production as factors that cause low CaCO3. Furthermore the new chronostratigraphy is an important foundation for future paleoceanographic studies. The ability to distinguish between primary production and dissolution is also important to establish a regional carbonate compensation depth (CCD). We report late Miocene to recent time series of X-ray Fluorescence (XRF) derived bulk sediment composition and mass accumulation rates (MAR) from eastern equatorial Pacific Integrated Ocean Drilling Program (IODP) Sites U1335, U1337, U1338 and Ocean Drilling Program (ODP) Site 849, and also report bulk density derived CaCO3 MAR at ODP Sites 848, 850 and 851. We use physical properties, XRF bulk chemical scans, and images along with available chronostratrigraphy to inter-correlate records in depth space. We then apply a new equatorial Pacific age model to create correlated age records for the last 8 Myr with resolutions of 1-2 kyr. Large magnitude changes in CaCO3 and bio-SiO2 (biogenic opal) MAR occurred within that time period but clay deposition has remained relatively constant, indicating that changes in Fe deposition from dust is only a secondary feedback to equatorial productivity. Because clay deposition is relatively constant, ratios of CaCO3 % or biogenic SiO2 % to clay emulate changes of biogenic MAR. We define 5 major Plio-Pleistocene Low CaCO3 % (PPLC) intervals since 5.3 Ma. Two were caused primarily by high bio-SiO2 burial that diluted CaCO3 (PPLC-2���1685-2135 ka, and PPLC-5���4465-4737 ka), while 3 were caused by enhanced dissolution of CaCO3 (PPLC-1���51-402 ka, PPLC-3���2248-2684 ka, and PPLC-4���2915-4093 ka). Regional patterns of CaCO3 % minima can distinguish between low CaCO3 caused by high diatom bio-SiO2 dilution versus lows caused by high CaCO3 dissolution. CaCO3 dissolution can be confirmed through scanning XRF measurements of Ba. High diatom production causes lowest CaCO3 % within the equatorial high productivity zone, while higher dissolution causes lowest CaCO3 at higher latitudes where CaCO3 production is lower. The two diatom production intervals, PPLC-2 and PPLC-5, have different geographic footprints from each other because of regional changes in eastern Pacific nutrient storage after the closure of the Panama Seaway. Because of the regional variability in carbonate production and sedimentation, the carbonate compensation depth (CCD) approach is only useful to examine large changes in CaCO3 dissolution. Tables SM-1 to SM-7: splice tables used for the 7 ODP and IODP drill sites in this study Tables SM-8 to SM-13: Chronostratigraphic depth ties among the drill sites. Tables SM-14 to SM-17: Age models for each drill site and age-depth ties at each site. Tables SM-18 to SM-23: scanning XRF data for 4 drill sites, and opal calibration data for Site 849 Tables SM-24 to SM-27: CaCO3 % estimated from Gamma Ray measured density for ODP Sites 848, 849, 850, and 851 Tables SM-28 to SM-34: Mass Accumulation Rates (MAR) for the 7 drill sites Tables SM-35 to SM-37: calculations of CCD from CaCO3 MAR
Astronomical tuning of sediment sequences requires both unambiguous cycle-pattern recognition in climate proxy records and astronomical solutions, and independent information about the phase relationship between these two. Here we present two different astronomically tuned age models for the Oligocene-Miocene Transition (OMT) from Integrated Ocean Drilling Program Site U1334 (equatorial Pacific Ocean) to assess the effect tuning has on astronomically calibrated ages and the geologic time scale. These alternative age models (from ~22 to ~24 Ma) are based on different tunings between proxy records and eccentricity: the first age model is based on an aligning CaCO3 weight (wt%) to Earth's orbital eccentricity, the second age model is based on a direct age calibration of benthic foraminiferal stable carbon isotope ratios (d13C) to eccentricity. To independently test which tuned age model and associated tuning assumptions is in best agreement with independent ages based on tectonic plate-pair spreading rates, we assign our tuned ages to the magnetostratigraphic reversals identified in deep-marine magnetic anomaly profiles. Subsequently, we compute tectonic plate-pair spreading rates based on the tuned ages. The resultant, alternative spreading rate histories indicate that the CaCO3 tuned age model is most consistent with a conservative assumption of constant, or linearly changing, spreading rates. The CaCO3 tuned age model thus provides robust ages and durations for polarity chrons C6Bn.1n-C6Cn.1r, which are not based on astronomical tuning in the latest iteration of the Geologic Time Scale. Furthermore, it provides independent evidence that the relatively large (several 10,000 years) time lags documented in the benthic foraminiferal isotope records relative to orbital eccentricity, constitute a real feature of the Oligocene-Miocene climate system and carbon cycle. The age constraints from Site U1334 thus provide independent evidence that the delayed responses of the Oligocene-Miocene climate-cryosphere system and carbon cycle resulted from highly nonlinear feedbacks to astronomical forcing.