University thesis:
Dissertation, Universität Bremen, 2023
Footnote:
Description:
The North Pacific has so far been insufficiently studied comparing to other oceans regarding paleoceanographic variations and its interactions with global climate. In particular, the present deep North Pacific acts as a CO2 sink, and the productivity regime, the carbonate system and other processes in relation to the carbon cycle are not well understood on long geological time-scales due to the lack of available proxies and records. This thesis aims to improve the understanding of the role of the North Pacific Deep Water (NPDW) in the Meridional Overturning Circulation and its sensitivity and response to upper ocean and atmosphere forcings and feedback mechanisms. Here, I present a series of new sediment records from a meridional transect along the Emperor Seamount Chain and on the Hess Rise in the NW Pacific. As a starting point of this thesis, an X-ray fluorescence (XRF)- and benthic foraminiferal isotope-based stratigraphic framework was developed for the sediment cores in this region. The subsequent proxy reconstructions comprise the last 550,000 years (550 ka BP) and focus on variations in paleo-productivity and the deep sea carbonate depositional environment, as well as deep water mass dynamics associated with carbon storage and outgassing. XRF-based productivity records indicate a southward decrease and an out-of-phase pattern in carbonate deposition below 40°N in the NW Pacific over the last 500 ka BP. It is assumed that regional shifts in the subpolar and subtropical gyre circulation, in close association with upper ocean stratification changes were the major factors determining the nutrient regime in this region. On the other hand, coherent interglacial increases in biogenic barium, carbonate flux, alkenone concentration, sand fraction and opal content in the subpolar region indicate that productivity had a more direct impact and depositional contribution on the carbonate system. The weak correlation between carbonate preservation and productivity on the subtropical gyre influenced Hess Rise, however, matched to the deep Pacific carbonate chemistry fluctuations which relate to carbonate dissolution. This mismatch also implies that the boundary shifted between the subpolar and subtropical gyres, which provided different nutrient regime to the NW Pacific region. Furthermore, I compared the stable isotope data from a monospecific benthic foraminiferal (Cibicidoides wuellerstorfi) record which reflects the NPDW to the source water masses in order to reconstruct the deep North Pacific ventilation history. A generally more stagnant glacial NPDW during the last four glacial-interglacial cycles is supported by our record. The 13C composition (Δδ13C) between NPDW and Circumpolar Deep Water suggests a steady southern water mass contributor, but a long-term change in the inflow strength into the North Pacific. During terminations II to V, a breakdown of the mid-depth to deep stratification in the NW Pacific was inferred by the abrupt decline in Δδ18O between NPDW and North Pacific Intermediate Water (NPIW), however, a similar signal in the ventilation was not clearly observed in Δδ13C. Based on Δδ13C signals, a rapid ventilation change during MIS 10 to 13 occurred from a relatively strongly ventilated interglacial NPDW to better ventilated NPIW during glacial. Taken together, these observations affirm the deep North Pacific as a substantial carbon reservoir via deep water circulation and upwelling during deglacial and interglacial periods. Lastly, I introduce a new integrated approach of using machine learning (ML) to quantify and predict geochemical data in sediments from XRF spectra. By using this approach, the reconstruction of the NW Pacific carbonate deposition patterns in future studies could be built on high-resolution quantitative data, without losing accuracy due to extensive laboratory work or machine bias.