東太平洋上新世到更新世浮游有孔蟲氧同位素和金屬及鈣比例重建之海水溫度分層

Abstract

東赤道太平洋（EEP）透過風驅湧升流與熱帶大氣環流耦合，對全球氣候具有關鍵影響。上新世暖期曾被提出可能存在類似聖嬰的「El Padre」平均態，但其在重大氣候轉型前後的上層海洋垂直結構演化仍不清楚。本研究利用 IODP Site U1338（3.6-2.58 Ma）浮游有孔蟲的δ¹⁸O 與 Mg/Ca 數據，結合不同棲位深度物種（T. sacculifer、N. dutertrei、G. crassaformis、D. altispira），重建混合層至溫躍層的溫度結構與鹽度相關分層，並探討其跨越北半球冰期增強（NHG，~2.7 Ma）的變化。 Mg/Ca 溫度結果顯示混合層相對穩定，而 NHG 之後上層溫躍層明顯降溫，使熱分層僅呈現小幅增強；整體溫差約為 5.8-14.3°C，顯示熱梯度並未出現長期崩塌，因此不支持持續的 El Padre 類平均態。相對地，結合 δ¹⁸O 與 Mg/Ca 計算的 δ¹⁸Oseawater 顯示 NHG 前後鹽度相關分層顯著增強，平均由 1.34 ± 0.09‰ 上升至 1.92 ± 0.10‰（p < 0.001），主要由混合層 δ¹⁸Oseawater 增加所驅動，而溫躍層變化較小。由於 δ¹⁸Oseawater 差值可能高估絕對鹽度差異，本研究將其解釋為混合層與溫躍層水文對比的相對變化。 與ODP Site 1241的比較顯示，NHG 之後 EEP 區域 δ¹⁸O 變化較一致，但在 MIS KM5c（~3.2 Ma）之前呈現較大空間差異，尤其在 N. dutertrei，暗示次表層水團來源與輸送路徑存在區域差異。此外，D. altispira 的 δ¹⁸O 值介於混合層與溫躍層物種之間，支持其棲位位於混合層下部，並可作為近表層替代指標。 總結而言，IODP Site U1338在上新世晚期時上層海洋分層的演化包含兩部分：溫躍層降溫造成的熱分層小幅增強，以及混合層水文改變主導的鹽度相關分層顯著增強。結果顯示 NHG 伴隨熱帶水文氣候與上層環流重組，並凸顯鹽度過程在海洋和氣候互動中的重要性。

The eastern equatorial Pacific (EEP) strongly influences global climate through wind-driven upwelling of cold, nutrient-rich waters and its coupling to tropical atmospheric circulation. During the Pliocene warm period, it has been proposed that the equatorial Pacific experienced an El Niño-like “El Padre” mean state, yet the evolution of upper-ocean vertical structure across major climate transitions remains incompletely constrained. Here we present paired oxygen isotope (δ¹⁸O) and magnesium-to-calcium (Mg/Ca) records from planktonic foraminifera at IODP Site U1338 (2°30’N, 117°58’W; 4200m water depth), spanning 3.6-2.58 Ma. Species occupying different depth habitats, Trilobatus sacculifer (mixed layer), Neogloboquadrina dutertrei (upper thermocline), Globorotalia crassaformis (sub-thermocline), and the extinct shallow-dwelling Dentoglobigerina altispira, allow reconstruction of Late Pliocene vertical temperature structure and salinity-related stratification across the onset of Northern Hemisphere Glaciation (NHG, ~2.7 Ma). Mg/Ca-derived temperatures indicate that mixed-layer conditions remained relatively stable, whereas upper-thermocline temperatures cooled significantly after the NHG, producing only a modest strengthening of thermal stratification. Across the full study interval, mixed-layer to thermocline temperature differences range from ~5.8 to 14.3°C, suggesting that the upper-ocean thermal gradient at IODP Site U1338 remained within a physically plausible envelope and did not undergo a sustained collapse consistent with persistent El Padre-like conditions. In contrast, reconstructed seawater δ¹⁸O (δ¹⁸Oseawater), calculated by combining planktonic δ¹⁸O with Mg/Ca temperatures, reveals a pronounced strengthening of salinity-related stratification across the NHG. Mean δ¹⁸Oseawater-based stratification increased from 1.34 ± 0.09‰ before the NHG to 1.92 ± 0.10‰ afterward (p < 0.001), driven primarily by higher mixed-layer δ¹⁸Oseawater values while thermocline δ¹⁸Oseawater remained comparatively stable. Because δ¹⁸Oseawater offsets can overestimate absolute salinity differences, this stratification signal is interpreted mainly in a relative sense as changes in mixed-layer-thermocline hydrographic contrast rather than a direct multi-psu salinity gradient. Comparison with ODP Site 1241 (Panama Basin) indicates broadly coherent δ¹⁸O variability across the eastern equatorial Pacific after the NHG, but greater spatial heterogeneity prior to MIS KM5c (~3.2 Ma), particularly in the thermocline-dwelling N. dutertrei, consistent with regional differences in subsurface water-mass properties and advection pathways. In addition, δ¹⁸O values of D. altispira fall between those of T. sacculifer and N. dutertrei, supporting a habitat in the lower mixed layer and its potential use as a near-surface proxy in intervals where conventional mixed-layer taxa are absent or poorly preserved. Overall, our results demonstrate that Late Pliocene upper-ocean stratification at IODP Site U1338 evolved through two distinct components: modest thermocline-driven strengthening of thermal stratification and a substantial increase in salinity-related stratification dominated by mixed-layer hydrographic change. These findings suggest that tropical hydroclimate reorganization and upper-ocean circulation changes accompanied the onset of NHG, providing new constraints on EEP water-column structure during a key climate transition and highlighting the importance of salinity-related processes in modulating ocean-climate interactions.