Research Topics and Methods

Objective: Time Series of Crustal Thickness Variations (Geophysical Surveys)

This project component focuses on mapping oceanic crustal thickness variations over climatic timescales using high-resolution geophysical methods. Acoustic techniques will establish the depth of the oceanic basement and Moho interface, while magnetic data will help relate crustal thickness changes to sea level. Bathymetric data, gathered via the Kongsberg EM122 12-kHz echo-sounder, will be compared to these thickness variations to assess any correlation with seafloor topography (e.g., abyssal hills).

Seismic reflection data, recorded by a 380 m streamer, will capture the volcanic basement depth, with sediment cover estimated between 0-200 m. Seismic refraction profiling will determine seismic velocities and Moho depth variations across closely spaced instruments, resolving crustal thickness changes of ~210 m over short distances. Sea surface magnetic field data, corrected for field variations, and shipboard gravity data, corrected for drift, will provide further insights into seafloor age and crustal structure.

Work Plan:
Refraction seismic data will be recorded and processed in SEG-Y format, with positioning refined by GPS time corrections. Signal quality will be improved via deconvolution and frequency filtering. Travel times of key seismic phases will be used for forward modeling, with tomographic imaging yielding a detailed 2D velocity structure. Shipboard gravity data will allow for mantle Bouguer anomaly calculations after corrections for water depth, sediment cover, and crustal structure, with density contrasts calculated from the seismic data. These anomalies will inform crustal thickness variations, providing insights into the relationship between crustal structure and climate-related processes.

The project aims to investigate changes in hydrothermal activity and their impact on sediment composition near mid-ocean ridges. Variations in melt production are expected to influence hydrothermal activity, which should be recorded by the deposition of metals such as Fe, Mn, Cu, Pb, and As in near-ridge sediments. Studies on ridges with different spreading rates have shown that peak hydrothermal inputs predominantly occurred during glacial terminations, especially during sea-level lowstands and delayed melt migration.

To clearly identify hydrothermal influences, the project will analyze lead (Pb) and neodymium (Nd) isotope compositions in sediments. Pb isotopes are considered reliable indicators of hydrothermal contributions, as they do not respond to redox conditions and have distinctive values near hydrothermal sources. In contrast, Nd isotopes are unaffected by hydrothermal inputs and instead provide insights into changes in deep water circulation. The project aims to reconstruct the evolution of hydrothermal inputs over time by integrating these isotope records with sedimentary hydrothermal metal concentrations and fluxes.

Goal: Time Series of Hydrothermal Activity in Sediments

Collecting Sediment Profiles:
To analyze changes in hydrothermal activity and MORB glass composition, sediment cores will be taken at selected Mid-Ocean Ridge (MOR) segments using systematic gravity and multi-coring along profiles perpendicular to the ridge. Cores are spaced to cover intervals of approximately 50,000 years, with precise locations determined through seismic and bathymetric surveys.

Establishing Age Models for Sediment Cores:
Age models are created through core correlation, alongside absolute and relative dating. Physical properties and color measurements are recorded immediately after core retrieval, while X-ray fluorescence scans aid in core correlation. This comprehensive data enables a continuous sediment time series above the basaltic basement.

Reconstructing Hydrothermal Activity:
Using metal contents and isotope analyses (e.g., lead and neodymium isotopes), we reconstruct hydrothermal activity patterns over time. These data help understand how hydrothermal inputs and bottom water circulation impacted the sediment composition.

Work Plan:
All cores undergo detailed logging and scanning to develop precise chronological frameworks. Selected reference cores will be dated using oxygen isotope stratigraphy and radiocarbon dating. These time series provide valuable insights into the relationships between sea level, water chemistry, and hydrothermal and magmatic processes at Mid-Ocean Ridges.

Objective: Analyzing Major and Trace Elements in a 1.5 Ma MORB Glass Time Series

Sampling and Analysis:
To construct the time series, each core will be sampled every 2 cm, with multiple glass fragments extracted from each section. If the sediment layer contains up to 80 cm of sediment-hosted glass (SHG), this approach could yield 200-400 glass samples per core—equivalent to the sample volume from a typical dredging or wax coring expedition, greatly increasing the analytical load for this project. The glass fragments will be mounted in polycarbonate disks with the Harvard in-house standard VE32 to ensure data consistency over the project's duration and comparability with prior work. Using laser ablation ICP-MS, samples will be analyzed with techniques optimized for high precision and throughput. Each sample will have two analysis points to control for phenocryst contamination. Since MOR glasses have low volatile content, major elements will be analyzed independently by normalizing to a fixed total minus inferred volatiles (typically under 0.3% for MORBs). Fifty-two trace elements will be measured with a precision of less than 3% for concentrations above 0.1 ppm. This setup enables the analysis of 50 samples per day, providing the foundational data for the time series.

Need for New Equipment:
With potentially 50,000 samples (~1,000 days of analytical work), the project exceeds the current ICP-MS lab capacity at Harvard, which is already heavily utilized. A dedicated new instrument is necessary to meet the project’s demands, with the added benefit of increased sensitivity, allowing for a broader range of elements analyzed at high precision.

Work Plan:
The new ICP-MS will be ordered within six months of the project start, followed by a six-month period for installation and calibration. As samples from the first cruise become available, major and trace element analyses will begin to investigate differentiation processes. Elemental ratios (e.g., La/Sm, Nb/Zr) will help assess melting variations, while ratios of highly incompatible elements (e.g., Th/U, Rb/Nb, Nb/Ta) may reveal source variations but could also reflect recent low-degree melting. Isotope data will be needed to separate these effects from long-term source variations, providing essential baseline data for further isotope analyses. The sediment stratigraphy studies will establish the timeline for these compositional changes, and comparisons with sediment compositions will help identify links to hydrothermal activity over time.

Objective: Radiogenic Isotope Variations in a 1.5 Ma MORB Time Series

Radiogenic Isotope Analysis (Sr-Nd-Pb-Hf):
Isotope analyses will be performed at GEOMAR, focusing on fresh glass and rock samples from wax, gravity core, and drilling samples. Due to the limited quantity of sediment-hosted glasses (SHG) and their depleted composition, high-precision bulk isotope analysis is necessary, as laser ablation methods are insufficient for detecting small radiogenic variations. For example, distinct geochemical intervals were identified in the Ferguson et al. (2017) core, with compositionally uniform glass intervals pooled to reach a 10 mg threshold. Each core will yield approximately four samples for analysis, resulting in an estimated 200-250 analyses per study area and 650-750 in total, or 130-150 analyses per year.

Challenges and Requirements:
Precise radiogenic isotope analysis of small glass samples requires very low contamination, high-purity processing, and minimal measurement error. The newly available Neoma MC-ICPMS, featuring advanced amplifier setups, is ideal for this work. For proper sample dilution before MC-ICPMS, 5% of processed samples will be measured using a quadrupole ICPMS (e.g., Thermo Scientific iCAP-TQ) to ensure the correct amplifier setup.

Work Plan:
Within six months, the instruments will be ordered, followed by installation, calibration, and development of ultra-pure analysis protocols over the next 12 months. Sample preparation will begin with well-characterized MORB glasses to refine procedures for small sample sizes, with isotope analyses of glass and rock samples continuing over the project's six-year span.

Volcanological Study of Glass Shards and Ash Layers:
A high-resolution study of glass shards (cryptotephra) in sediment cores will track temporal changes, analyze formation mechanisms, and map distribution. Samples from ~10 cores per transect will be analyzed, capturing data across glacial cycles. Distinct ash layers, chemically differentiable, will serve as stratigraphic markers, refining age models and providing insights into mantle heterogeneity. Scanning electron microscope (SEM) analysis will characterize glass shard fragmentation processes, distinguishing magmatic from water-driven fragmentation.

Work Plan:
All SHG and ash layer samples will be sieved, with grain size distributions analyzed. Glass shards will be studied for microlite populations and bubble shapes using SEM, followed by electron microprobe analysis (EMP) at GEOMAR. These analyses will yield detailed data on size, morphology, and composition, helping to correlate eruption events along the transect.

Summary:
This project will produce a stratigraphically controlled dataset of glass shard characteristics, bubble morphology, chlorine content, and tephra markers, enabling a comprehensive record of eruptive events and their processes along the study transects.