60th Annual Report on Research 2015

Project Background

Diatoms are responsible for as much as 75% of the total primary productivity in the oceans; silicic acid utilization and the precipitation of biogenic silica (bSiO₂) as diatom frustules is the most significant component of the marine silicon cycle in the modern ocean. Silicon and oxygen isotope values in bSiO₂ are increasingly used as a chemostratigraphic tool for characterizing global biogeochemical cycles; however, the effects of diagenetic processes (e.g. dissolution, dehydroxolization, precipitation, and phase changes from opal-A to Opal-CT) on these isotope proxies are not well understood. The research goals of this project are twofold: 1) to conduct a suite of experiments that will examine the fractionation of silicon and oxygen isotopes during sedimentation and diagenesis of bSiO₂, then 2) to apply our understanding of isotopic variations in bSiO₂ to develop records of silicon and oxygen isotope variations in a previously collected Antarctic marine sediment core (AND-1B). The ultimate goal of this project is to improve our understanding of global silicon cycling and elucidate large-scale changes in marine nutrient cycling over geologic time.

Research Progress: Experimental Results The experimental focus of this research is on quantifying variations in silicon and oxygen isotope values as bSiO₂ undergoes diagenesis in laboratory settings designed to approximate sedimentary environments and geologic timescales. To this end, two species of marine diatoms (Stephanopyxis turris and Thalassiosira weissflogii) have been cultured in a controlled laboratory environment where growth conditions were continuously monitored. SEM images of silica frustules from cultured S. turis are shown in Fig. 1.

                             
The bSiO₂ was then aged in a 0.7 M NaCl solution at a pH of 8.0 at 85 degrees C for 37 days. Fourier transform infrared (FTIR) spectra were used to examine changes in the water content in purified cultured and aged diatom frustules. FTIR spectra data on the modes of Si-O and O-H species were collected on powdered diatom samples using a Mattson ATI Genesis Series FTIR equipped with a Pike MIRacle attenuated total reflection device at Northern Illinois University. FTIR spectra of S. turris frustules demonstrate a significant reduction in the abundance of silanol groups (Si-OH) associated with dehydroxylation of the biogenic silica (Fig. 2). Silanol abundance is calculated as the ratio of the integrated area under the 950 and 800 cm^-1 peaks (gray bars). Surface charge density of the diatom silica was measured using acid-base titrations performed at a pH endpoint of 7.5 as a measure of surface silanols (Si-OH). The surface charge density, and therefore the number of surface silanols also decreased during the 37-day aging experiment.                                                  
Oxygen isotope values also decreased rapidly during the initial stages of the aging experiment (Fig3a); however, the decrease does not seem to correlate with the reduction of silanol density during subsequent dehydroxylation (Fig 3b). Oxygen isotope values of fresh cultured bSiO₂ (d¹⁸O = 31.8; ± 0.54; n = 6; open square) are in close agreement with the silica-water fractionation relationship for other cultured/fresh diatom silica; however, aged samples have significantly lower oxygen isotope values (ave. d¹⁸O = 23.8± 0.53; n=19; closed squares). Converting the surface charge and silanol abundance to silanol density (%) provides a clearer view of the dehydroxylation dynamics (Fig. 3b). The initial ratio of initial internal to external silanols theoretically constrained at 10:1. Combined internal and external silanol density (closed circles/solid line; FTIR), and external only silanols (open circles/dashed line; titration) decrease throughout the aging experiment. The internal silanols (broken line), calculated as the difference between total and external results, represent the largest component of the total silanols throughout the experiment. These results suggest that following the initial diagenetic reaction, a decoupling of the dehydroxylation process and isotopic variations during early stages of diagenesis in bSiO₂. A likely explanation for both the rapid decrease in silanol density and oxygen isotope values is dissolution of the bSiO₂ and precipitation of non-bSiO₂ during the initial phase of the experiment.                                                                  
Future work and Research Impact

            Silicon isotope measurements are underway on the cultured and aged bSiO₂. Additional experiments are also aimed and quantifying variations in oxygen and silicon isotope values of the bSiO₂ during phase changes from opal-A to opal-CT at elevated temperatures. This research has fostered the academic development two graduate and will result in two master's theses and numerous peer review publications. Additionally, understanding the relationship between chemical, structural, and isotopic variations in bSiO₂ is a major earlier career research goal for the PI. Using oxygen and silicon isotope analyses as a proxy for past environmental conditions has the potential to greatly expand our understanding of nutrient cycling, ocean circulation, and climatic variations in high latitude marine environments. These experimental data represent a critical step in interpreting oxygen and silicon isotope values of bSiO₂ from the AND-1B core and other marine sediment records.