UC Santa Cruz

This thesis explored the use of bulk stable nitrogen (δ15N) and carbon (δ13C) isotope records from central Pacific proteinaceous deep-sea corals to create novel multi-millennial scale, sub-decadal resolution records of export production in this ocean region. I used live collected and subfossil coral specimens from both the central North Pacific Subtropical Gyre and Line Islands, to explore how coral isotope records can reconstruct climate forced changes in oceanography in very different areas of the Pacific, as well as to explore the limits of isotope records and coral skeleton preservation over time.

My thesis is organized in three parts. In the first chapter, I used three Kulamanamana haumeaae specimens from the main Hawaiian Islands to create ~5000 year record of North Pacific Subtropical Gyre (NPSG) export production δ15N and δ13C history, greatly expanding previous coral records (by ~4000 years) into the Mid-Holocene. These bulk isotope records revealed a very different oceanic regime during the mid-Holocene, with higher δ15N values indicative enhanced flow of source waters bearing signatures of denitrification from the eastern Pacific. Further, these new δ15N data confirmed that the extremely low present day δ15N values recorded by deep sea corals (∼8‰) are unprecedented for the NPSG, at least within the past five millennia. My data also suggested that the coupling or decoupling of δ15N and δ13C values may be an important new tool for understanding the ocean’s response to climate shifts. During most biogeochemical regimes shifts in this period, δ15N and δ13C trends were synchronous (similar to recent coral records), consistent with nutrient concentration and isotopic values acting together with plankton community shifts. However, several distinctly decoupled regimes also appear, indicating unusual mechanisms must have been at work. Phytoplankton species composition and nutrient source changes are hypothesized as the dominant mechanisms which controlled the coupling and decoupling of δ15N and δ13C values, likely primarily influenced by changing oceanographic conditions (e.g., stratification versus entrainment).

Chapter 2 was a methodological study, aimed at understanding the extent in time over which proteinaceous coral records from this region can be extended. Specifically, this study examined the relative changes due to degradation in subfossil corals, asking to what degree bulk versus compound-specific isotope values remain viable over multi-millennial time scales. This chapter examined one of the oldest deep sea K. haumeaae corals ever collected and the specifics of proteinaceous skeleton preservation over ~10 kyrs on the seafloor. I compared amino acid (AA) molar composition and isotope pattern changes, together with bulk δ13C and δ15N values in a subfossil (~9.6-11.6 kyrs BP) coral skeleton with a live collected specimen from the same region (Cross Seamount, Hawaii). I aimed to understand the effects of long-duration benthic oxic exposure on AAs and organic preservation in a subfossil coral. This study shows that while bulk isotope values are dramatically altered in a clear diagenetic outer zone, likely due to carbonate deposition and protein-structural degradation, most of the AA isotopic data remains almost entirely unaltered. Some notable results included nearly ~50% of glycine loss over ~10 kyrs, likely due to abiotic water based hydrolysis of the protein matrix, which nevertheless does not appreciably shift glycine’s isotope values. Despite changes to molecular composition, compound-specific isotope analysis of amino acids (CSIA-AA) data patterns remained almost entirely unchanged in inner coral layers. This indicates that in contrast to bulk isotope data, CSIA-AA data can reconstruct paleo-oceanographic biogeochemical and ecosystem information in subfossil corals beyond the clear outer diagenetic horizon, which is easily identifiable from an evaluation of C/N ratios and the ΣV degradation proxy.

My last chapter, switched back to paleo-oceanographic application for a very different region in the equatorial Pacific (Kingman Reef, Line Islands), one for which no prior proteinaceous coral records exist for the most recent two millennia. I examined the bulk δ13C and δ15N values and CSIA-AA δ15N values of both a live-collected and subfossil specimen to determine how export production isotope values have changed in response to climate shifts through the last ~2000 years. I proposed shifts in both δ13C and δ15N values are fundamentally driven by changing current dynamics, with an alternation between dominance by two water sources: western Pacific waters in the North Equatorial Countercurrent, and east Pacific waters in the North Equatorial Current with distinctly different nutrient concentrations and isotopic signatures. I utilized modern day El Niño Southern Oscillation patterns as a model to interpret the overall data, e.g. lower δ13C and δ15N values are indicative of more El Niño-like (warmer/wetter) conditions, with smaller-celled prokaryotic phytoplankton communities, and advection from the west Pacific. These changes in current dynamics recorded in my new records and are likely related to shifts in the Intertropical Convergence Zone, which is the major source of ocean stratification due to controls on precipitation and climate in the region.

Overall, by combining both new methodological work utilizing CSIA-AA to determine preservation state, and a novel set of coral archive records, my thesis expanded the utility of and potential application of deep sea coral archives, while specifically greatly expanding the biogeochemical history of oceanic change in two separate central Pacific regions. My thesis examined time periods and regions for which no such prior records exist, showing that both modern and subfossil specimens can provide high resolution information of Pacific Ocean variability through the Holocene.