61st Annual Report on Research 2016

This project started in August 2015 with a visit to the Alaska Geologic Materials Center in Anchorage to collect borehole samples from the Gulf of Alaska that form the basis for our research project. This short visit to Alaska was a great start for graduate student Nathaniel Bootes into his Master's thesis research. Nate has never been in Alaska before and so we spent two extra field days to explore the exciting tectonics and geology just outside Anchorage. This trip was not only a good opportunity to get to know each other, but also to connect with our collaborator Dr. Lease from the USGS in Anchorage, who helped prepare the samples at the storage facility and shipped them to the University of Cincinnati in Ohio.

Over the past year Nate has been incredible busy in the lab conducting mineral separation for extracting zircon and apatite and subsequently preparing the separates for fission track (FT) dating. These are all very time consuming steps and undergraduate student Victoria Thomas helped with the mineral separation as well as with grinding, polishing, and etching of the apatite FT mounts.

In total, 22 samples from 4 different boreholes yielded zircons. During the past spring (2016) I trained Nate in conducting zircon FT dating in the laboratory at Cincinnati and he spent the summer counting his samples. Because our samples are from sediment, each grain has a different FT age. The interpretation is therefore not based on individual grain ages, but on age populations. The general goal is to analyze ca. 100 grains per sample to obtain a statistically meaningful age distribution. This was accomplished for 16 samples. Due to the limited amount that we were able to collect for some boreholes, and the very fine-grained nature of the sediment, some samples did not yield enough zircons. For three samples we analyzed >50 grains and three samples <30 grains. The focus of the project is primarily on the younger <8 Ma deposits (14 samples) that can be linked to the development of the St. Elias Mountains, whereby the remaining eight samples are from older strata deposited between 40 and 18 Ma and provide insight on the northward translation of the Yakutat plate along the North American margin.

For our interpretation of the exhumation signal from sediment sources on land, it is crucial to identify and exclude volcanic material that would provide a cooling age due to magmatism and not due to rock exhumation. We therefore prepared road maps of our samples that allow us to identify each grain that has been previously dated for fission-track dating. At the LaserChron Center at the University of Arizona in Tucson we conducted U-Pb dating of the zircon grains. The double dating analysis allows us to assign each grain with both its FT and U-Pb age. The double dating not only permits identification of volcanic grains (FT = U-Pb age) but also facilitates provenance analysis.

All zircon dating analyses were finished over the summer and Nate has all his data, in total 1963 zircon FT ages of which 1876 where also analyzed for U-Pb dating.

Apatite FT dating was not in the proposed research project, however, due to the good yield of apatite grains we prepared 19 samples for FT dating. Undergraduate student Victoria will be trained this fall semester in conducting apatite FT dating on these samples in the course of her capstone project.

Since our return from the LaserChron Center Nate has been analyzing the data and preparing plots. He submitted an abstract and will present this research at the AGU Fall meeting in December 2016. I submitted an abstract and will present the offshore sediments in the Gulf of Alaska that will tie together our new data of this project and those produced from the IODP Exp 341 boreholes that were drilled in 2014 in the same area.

The data from the two projects are very complimentary because both cover boreholes drilled on the Yakutat shelf in the Gulf of Alaska. The IODP boreholes are only 1 km deep and material recovered was deposited very recently, <0.8 Ma due to the extreme high glacial sedimentation rates. In contrast the ACS-PRF project uses industry boreholes that cover sediment deposited deeper in the past.

This research project is a new direction for me because I am using the tools of low-temperature thermochronology on sedimentary deposits to study exhumation pattern of the past and link the sediment source to its offshore sink. All my previous research has focused on land, investigating the spatial and temporal pattern of evolving convergent plate margins by analyzing bedrock or modern glacial deposits.

At this stage of the project it is too early for any conclusions, but I am very pleased about the solid dataset we have produced now and how Nate gets excited as he digs deeper into understanding his data. During the past weeks we started to have good discussions about interpretation. Our next goal is to conduct all data analysis and plotting before Dr. Lease visits the department at the beginning of November. This will be a good opportunity for Nate to present his data to Dr. Lease and get more feedback before he prepares for AGU and writing his Master's thesis.

The goal for the second year of the project is to prepare a manuscript that will be submitted to a peer-reviewed journal and Nate's graduation. I am planning for a second manuscript together with Victoria using the apatite FT data.

Figure: top: map of southeast Alaska with main tectonic units and the borehole location in the Gulf of Alaska. Bottom: Stratigraphy of the four boreholes investigated displaying the distribution of the sample with respect to stratigraphy.