B--Sources Sought - Geochronology

Page 1 of 3 US Geological Survey Alaska Science Center Statement of Work I - General Information: A - Introduction: [The USGS Alaska Basins and Petroleum Systems (hereafter, ABPS) project requires high resolution geochronology for volcanic ash (hereafter tephra) samples from outcrop stratigraphic sections and research cores through Cretaceous marine strata on the North Slope of Alaska. The absolute age of the strata remains unclear, but it likely spans 10s of Myr of geologic time and dates to about 130–90 Ma. Determination of the absolute age of the tephras is critical for interpretation of the petroleum source rock geology of the Alaska North Slope province as well as the development of palaeoceanographic records of Cretaceous greenhouse periods in the western Arctic that produced these prolific source rock intervals.] B - Background: [The sections and cores described above were collected as a result of a multi-year effort, directly mandated by the USGS Energy Resources Program. This effort is in support of longer-term USGS efforts to better understand the petroleum resource endowment of the North Slope province. It was specifically motivated by recent oil and gas discoveries in Cretaceous strata in nearby parts of the North Slope Province, at least some of which are likely sourced by strata equivalent to those in the aforementioned sections and cores. These represent the world’s largest onshore oil discoveries in decades and are therefore of great importance to both the State of Alaska and the energy security of the United States of America. The ABPS team has also recently established pioneering marine geological and geochemical records of Cretaceous greenhouse periods along the paleo-Arctic continental margin of Alaska. The stratigraphic record in these sections will be critical for further development of these records across a paleogeographic gradient that spans the continental shelf, slope, and basin floor.] C - Scope: [The awardee must 1) separate zircon from seven bulk tephra samples, 2) conduct U-Pb geochronologic screening using laser-ablation inductively-coupled-plasma mass spectrometry (hereafter, LA-ICP-MS), and 3) based on the results of the screening analysis and in consultation with ABPS representatives, develop and execute a plan to conduct high- accuracy, high-precision U-Pb geochronology using chemical-abrasion isotope-dilution thermal-ionization mass spectrometry (hereafter, CA-ID-TIMS). The awardee should discuss geological interpretation of these results with ABPS representatives, with the specific focus of establishing a) ideally a depositional age, or else b) a maximum depositional age for each of the seven tephra bed samples. II - Work Requirements: A - Technical Requirements: -- 1 of 3 -- Page 2 of 3 1 - [In order to determine ages with sufficiently good accuracy and precision for our purposes, we require a three-step workflow. First, we require separation of zircon with unknown ages (hereafter, “unknowns”) from bulk tephra samples. The mineral separation process must be oriented towards preserving any zircon and should yield 10s of zircons per sample at a minimum. The analyst must be able to document successful zircon recovery from tephra with a track record of peer-reviewed publications. Second, we require initial geochronologic screening of grains by LA-ICP-MS; this is to identify the presence of any volcanic zircon that were either a) directly deposited within the stratigraphic section via airfall and settling through the water column, or else b) recycled into the stratigraphic section shortly after deposition as described above. Finally, we require follow-up high precision/accuracy geochronology by CA-ID-TIMS, with an analytical plan that is specifically informed by step 2 and vetted by ABPS staff. The analysts should assess the ages of at least four or five unknowns using both the 238 U/ 206 Pb and the 235 U/ 207 Pb systems. Analysis of paired isotopic systems is standard practice in the geochronology committee and facilitates understanding of the geochronologic system in the unknowns. Critically, the high-resolution mass spectrometry in step 3 must be able to reproduce ages of appropriate (e.g. Mesozoic-Cenozoic) geochronologic reference materials to within 104 –105 yrs. The reproducibility of reference material ages should be demonstrably applicable to the analytical session in which the unknowns described above are analyzed. Analytical uncertainties on unknowns in step 3 must also be on the order of 104 –10 5 yrs. Such high accuracy and precision is critical for a number of reasons, but we specifically stress here the need to correlate paired stable carbon isotope stratigraphy’s from the sections/core to global reference records. Both chemical abrasion and isotope dilution methods are required to achieve these levels of accuracy and precision, and to ensure the highest data quality, isotope tracers used during the analytical process should be certified by an internationally recognized geochronological organization (e.g., Earth Time). The levels of accuracy and precision described above are critical and the awardee must have a demonstrable track record of producing data of this quality in peer-reviewed literature. We also require the analysts to measure Th/U ratios in unknowns, and to assess common Pb inventories in unknowns. We require technical reports with measured U-Pb isotope ratios (and analytical uncertainties on them) that are adequate to assess concordance, we require reporting of a preferred age (and analytical uncertainties), we require documentation of paired analyses on geochronologic reference materials, and we require clear documentation of all analytical protocols. We require discussion with the lab analysts on the appropriate geological interpretation of the geochronological data.] 2 - Data reports will be reviewed by ABPS project staff to ensure that criteria above in item II.A.1 are satisfied. B - Deliverables: 1 - The deliverables sh…