Late Pliocene Cordilleran Ice Sheet development with warm northeast Pacific sea surface temperatures

Abstract

Abstract. The initiation and evolution of the Cordilleran Ice Sheet are relatively poorly constrained. International Ocean Discovery Program (IODP) Expedition 341 recovered marine sediments at Site U1417 in the Gulf of Alaska (GOA). Here we present alkenone-derived sea surface temperature (SST) analyses alongside ice-rafted debris (IRD), terrigenous, and marine organic matter inputs to the GOA through the late Pliocene and early Pleistocene. The first IRD contribution from tidewater glaciers in southwest Alaska is recorded at 2.9 Ma, indicating that the Cordilleran Ice Sheet extent increased in the late Pliocene. A higher occurrence of IRD and higher sedimentation rates in the GOA during the early Pleistocene, at 2.5 Ma, occur in synchrony with SSTs warming on the order of 1 ∘C relative to the Pliocene. All records show a high degree of variability in the early Pleistocene, indicating highly efficient ocean–climate–ice interactions through warm SST–ocean evaporation–orographic precipitation–ice growth mechanisms. A climatic shift towards ocean circulation in the subarctic Pacific similar to the pattern observed during negative Pacific Decadal Oscillation (PDO) conditions today occurs with the development of more extensive Cordilleran glaciation and may have played a role through increased moisture supply to the subarctic Pacific. The drop in atmospheric CO2 concentrations since 2.8 Ma is suggested as one of the main forcing mechanisms driving the Cordilleran glaciation.

Group 2i Isochrysidales produce characteristic alkenones reflecting sea ice distribution

“…Validation of %C 37:4 as paleo sea ice proxy in downcore sediments. Elevated %C 37:4 has long been observed in concordance with other sea ice proxies, such as IRD [45][46][47][48][49] , diatoms 48,50 , foraminifera assemblages, 24,51 and IP 25 52 in sediment records spanning the Late Pliocene to the Holocene. However, the interpretations of %C 37:4 were essentially derived from long-term SSS change instead of the occurrence of sea ice in previous studies (Supplementary Table 1).…”

Orbital and Suborbital‐Scale Variations of Productivity and Sea Surface Conditions in the Gulf of Alaska During the Past 54,000 Years: Impact of Iron Fertilization by Icebergs and Meltwater

“…To determine SSTs using the alkenone unsaturation index $${\mathrm{U}}_{37}^{\mathrm{K}\prime }$$, alkenones were analyzed by gas chromatography with chemical ionization mass spectrometry (GC‐CIMS), following the instrument method described in detail by Sánchez‐Montes et al. (2020). The $${{\mathrm{U}}_{37}^{\mathrm{K}}}^{\prime }$$ index is calculated from the relative abundances of the di‐ and tri‐unsaturated C 37 methyl alkenones as defined by Prahl and Wakeham (1987): $$${\mathrm{U}}_{37}^{\mathrm{K}\prime }=\frac{({\mathrm{C}}_{37:2})}{({\mathrm{C}}_{37:2}+{\mathrm{C}}_{37:3})}$$$ …”

“…To determine SSTs using the alkenone unsaturation index 𝐴𝐴 U K′ 37 , alkenones were analyzed by gas chromatography with chemical ionization mass spectrometry (GC-CIMS), following the instrument method described in detail by Sánchez-Montes et al (2020). The 𝐴𝐴 U K…”

“…′ index is calculated from the relative abundances of the di-and tri-unsaturated C 37 methyl alkenones as defined by Prahl and Wakeham (1987): Although alternative high-latitude SST calibrations are also available (Supporting Information S1) the same trends are recorded regardless of calibration. Using multiple extractions and analyses of a sediment sample used as a laboratory internal reference from the South Atlantic, the precision of the measurements (±1σ) was calculated to be better than 0.003 𝐴𝐴 U K′ 37 units (or 0.1°C) (Sánchez-Montes et al, 2020).…”

Alkenones as a temperature proxy in fin whale (Balaenoptera physalus) tissues

“…The analysis of alkenones, which has been widely used in paleoceanography, is an effective tool to measure environmental water temperature (e.g., McCaffrey et al 1990; Kennedy and Brassell 1992; Brassell et al 2004; Bendle and Rosell‐Melé 2007; Lourenço et al 2016; Sánchez‐Montes et al 2020). Alkenones are highly resilient molecules composed of a group of di‐, tri‐, and tetra‐unsaturated long‐chain ketones, (de Leeuw et al 1980) that remain in the sediments for millions of years with little degradation (Brassell et al 2004).…”