Radiolarians and silicoflagellates as clues of the paleoceanographic history of Guaymas Basin in the Gulf of California during the past 31,000 yr
Authors: Velázquez-Aguilar Mauricio , Pérez-Cruz Ligia , Urrutia-Fucugauchi Jaime , Aldama-Cervantes Alejandro , Venegas-Ferrer Rafael , Aiello Ivano , Jiang Shijun , Ran Lihua , Sarao John , Martínez Priscilla , Váldez-Hernández Marysol , Expedition 385 Scientists International Ocean Discovery Program 
Published: February 2023

&lt;p&gt;We investigated the changes in water masses circulation patterns in the Guaymas Basin in the Gulf of California to reveal the oceanographic and climatic evolution. We analyzed silica microfossils: Polycystine Radiolaria (hereafter radiolarians), and silicoflagellates, in the sediments of the deep-sea cores from Site U1545-A (~42 mbsf) of the IODP Expedition 385, collected on the west side of the basin.&lt;/p&gt;&lt;p&gt;The age model is based on 17 AMS radiocarbon dates covering the last 31,000 yr, between MIS (Marine Isotope Stage) 3 and MIS 1. Sedimentation rates ranged between 0.88 to 2.02 mm/yr. The highest sedimentation rates (ranging from 1.61 to 2.1 mm/yr) are between &amp;#8764;31,000 and 19,000 yr (including the Last Glacial Maximum, LGM), coinciding with falling sea level and a maximum ice volume.&lt;/p&gt;&lt;p&gt;&amp;#160;A total of 122 samples were analyzed, and 145 radiolarian taxa and 16 silicoflagellate species were identified. Radiolarians are rich, diverse, and well-preserved; overall, diversity is much higher for the last 15,000 yr.&lt;/p&gt;&lt;p&gt;Factor analysis of radiolarian assemblages suggested regional oceanic patterns based on environmental affinities. The dominance of &lt;em&gt;Siphocampe lineata&lt;/em&gt;, intermediate dweller radiolarian, was evident in the transition of MIS 3 to MIS 2, suggesting a significant influence of the Pacific Intermediate Water prior to the Last Glacial Maximum (LGM) and in Heinrich-1 (H-1), probably due to stronger North Pacific Intermediate Water (NPIW) formation. Besides, during the LGM, surface radiolarians assemblage and silicoflagellate specie such as &lt;em&gt;Cycladophora davisiana&lt;/em&gt; and &lt;em&gt;Botryostrobus aquilonaris&lt;/em&gt;, and &lt;em&gt;Octactis speculum&lt;/em&gt;, supports the idea of an incursion of the California Current Water (CCW) into GB, possibly by the southward shift migration of the NPH (North Pacific High) system, caused by the polar jet displacement and, because of the weakened NW-winds as well as the strengthened poleward circulation into the gulf.&lt;/p&gt;&lt;p&gt;The assemblage of &lt;em&gt;Tetrapyle octacantha&lt;/em&gt; group and &lt;em&gt;Phorticium pylonium&lt;/em&gt; group, surface dwellers radiolarians, and the silicoflagellate &lt;em&gt;Dictyocha fibula&lt;/em&gt; var. &lt;em&gt;messanensis&lt;/em&gt; indicated the dominance of Tropical Surface Water related to warm conditions and the influence of tropical water incursions enhanced by the Costa Rica Current. This association becomes more evident during the BA (Bolling Allerod) and the Holocene, mainly during the HTM (Holocene Thermal Maximum).&lt;/p&gt;&lt;p&gt;Surface dwellers species (&lt;em&gt;Lithomelissa setosa&lt;/em&gt;, &lt;em&gt;L. thoracites&lt;/em&gt;, &lt;em&gt;Peridium longispinum&lt;/em&gt;, and &lt;em&gt;Octactis octonaria&lt;/em&gt;), associated with the Gulf of California Water prevailed in episodic intervals during the late Pleistocene and the Holocene, suggesting mixed-water species. Due to an increase in evaporation processes in the gulf caused by the strength of NW winds and winter convection in the gulf, the dominance of this water mass may have occurred synchronously with the Monsoon System shift during the Holocene.&lt;/p&gt;