Several modelling and observational studies suggest deep water formation in the subpolar North Pacific as a possible alternative mode of thermohaline circulation that occurred in the warm Pliocene, a time when global atmospheric partial pressure of carbon dioxide was like the modern atmosphere (~400 ppm). We test this hypothesis by measuring the δ¹³C of the benthic foraminifer Cibicidoides wuellerstorfi collected from northernmost Pacific mid-Piacenzian Warm Period (3.264–3.025 Myr ago) sediments. The data reveal progressively more isotopically negative dissolved inorganic carbon along a northward Equator-to-pole transect, the opposite of the expected Pliocene Pacific meridional overturning circulation signal. C. wuellerstorfi δ¹³C is also often more positive at the deeper Ocean Drilling Program (ODP) site 887 compared with the shallower ODP site 883, suggesting ‘bottom-up’ ventilation of the deep Pacific Ocean. We then present alkenone sea surface temperature and export-productivity data from ODP site 883, which suggest that late Pliocene subarctic North Pacific carbonate sedimentation was, at least in part, probably due to higher coccolithophore export production, rather than North Pacific Deep Water formation as previously argued. Therefore, we suggest it is unlikely that North Pacific Deep Water formation occurred in the mid-Piacenzian Warm Period, although a shallower overturning cell cannot be ruled out.

一些建模和观测研究表明，北太平洋副极地的深水形成可能是发生在温暖的上新世的温盐环流的一种可能的替代模式，当时全球大气二氧化碳分压类似于现代大气（~400 ppm）。我们通过测量从太平洋最北端皮亚琴兹暖期（3.264–3.025 Myr 前）沉积物中采集的底栖有孔虫Cibicidoides wuellerstorfi的 δ ¹³ C 来检验这一假设。数据显示，沿着向北的赤道到极地横断面，溶解的无机碳的同位素负值逐渐增加，这与预期的上新世太平洋经向翻转环流信号相反。与较浅的 ODP 站点 883 相比， C. wuellerstorfi δ ¹³ C 在较深的海洋钻探计划 (ODP) 站点 887 处也通常更正，这表明深太平洋的“自下而上”通风。然后，我们提供来自 ODP 站点 883 的烯酮海面温度和出口生产力数据，这表明上新世晚期亚北极北太平洋碳酸盐沉积可能至少部分归因于更高的颗石藻出口产量，而不是北太平洋深水形成。之前争论过。因此，我们认为北太平洋深水形成不太可能发生在皮亚琴兹暖期中期，尽管不能排除较浅的翻转单元。