Carbon cycle and climate fluctuations during the early Paleogene: Sedimentological characteristics and environmental ramifications

Abstract

The early Paleogene was marked by extensive changes related to Earth surface temperature, carbon cycling, and the hydrological cycle. This included at least two, and probably more, geologically brief (~200-k.yr.) intervals of extreme warming, the Paleocene-Eocene thermal maximum (PETM) and the Eocene thermal maximum-2 (ETM-2) along with a moderately-long (~1.5-2 M.yr.) period of warmth (i.e.; Early Eocene Climatic Optimum [EECO]). This was preceded by a moderately-long (~2 M.yr.) period of cool conditions (i.e. Paleocene Carbon Isotope Maximum [PCIM]) and followed by the initiation of long-term cooling through the Cenozoic. The long-term rise in warmth and numerous short-term “hyperthermal” events, marked by pronounced negative carbon isotope excursions and clay-rich horizons (Zachos et al., 2005; Nicolo et al., 2007), have been linked to massive injections of 13C-depleted carbon into the ocean-atmosphere system and intense global climate change, but the exact character of the hyperthermals is not well-recognized. To better constrain and understand their occurrences, well-resolved and high-resolution records across the entire interval of interest is necessary. Preceding studies demonstrated major fluctuations in carbon cycling and terrestrial weathering (e.g.; Nicolo et al., 2007) during the latest Paleocene and earliest Eocene as well as a significant drop of dissolved oxygen concentrations during the PETM onset (Nicolo et al., 2010). However, causes, environmental impact, and relationships relating carbon release and capture to terrigenous runoff and surface temperatures throughout the exogenic system, particularly in spatial and temporal contexts, remain unclear.

The southern Pacific region (Clarence Valley) and Indian Ocean (Ninetyeast Ridge) represent large realms of our planet’s ocean system that have been largely overlooked in part due to limited Paleogene core availability from Integrated Ocean Drilling Program (IODP). There is a need to better improve records related to carbon in the exogenic system throughout the Cenozoic, and in particular, the Paleogene. As of now, records lack in these regions.

Clarence Valley in eastern Marlborough, New Zealand trends southwest for ~80 km between the Seaward and Inland Kaikouras. Along the northwest margin is a succession of streams, including Mead and Branch Streams, which have incised and exposed an uplifted and rotated block consisting of Amuri Limestone, a calcareous-rich formation within the Muzzle Group. These outer shelf and upper continental slope strata originally accumulated as terrigenous detrital clay minerals, biogenic silica, and biogenic carbonate. This was situated contiguous to a neritic carbonate platform along a passive margin of proto-New Zealand at ~55-50°S latitude.

Ninetyeast Ridge, one of the longest near-linear features on Earth with a length of ~4600 km (from ~10ºN to ~31ºS), is located in the Indian Ocean. Numerous Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) sites have been drilled adjacent to or along the ridge crest. Three sites in particular drilled during DSDP Leg 22 (i.e.; Sites 213, 214, and 215) include calcareous-rich material from much of the Paleogene. These sediments can provide additional constraints to further our understanding of Indian Ocean paleoceanographic changes during the Paleogene.

The lack of data for the Paleogene has been an issue for quite some time now. As such, the Paleogene remains a specific interval of time that has the potential to be much better understood by integrating current views of carbon cycling to new and wellresolved data-sets. Here I analyzed sequences exposed in Clarence Valley and sections from Ninetyeast Ridge DSDP Sites to address these issues. These records were integrated into a global context to relate short-term (<100 k.yr.) and long-term (> 1 m.yr) changes. I generated lithologic and carbon isotopic records to evaluate the entire period of interest, including the PCIM, specific hyperthermals, EECO, and the Middle Eocene Climatic Optimum (MECO). Integration of each section revealed similarities, including long-term trends with similar carbon isotopic baselines and short-term events. Expanded marl-rich units concurrent to lower δ13C, specifically across CIEs, generally characterized marginal sedimentation whereas condensed intervals largely spanned deep-water settings, resulting from carbonate dissolution. Together, these studies indicate carbon addition and removal mechanisms repeatedly spanned much of the Paleogene, causing calcite compensation depth (CCD) fluctuations, related to lithologic, and carbon isotopic changes.