Time-transgressive Holocene maximum of temperate and Mediterranean forest development across the Iberian Peninsula reflects orbital forcing

Abstract

We describe a new pollen and Sea Surface Temperature (SST) record with subcentennial to centennial resolution from a well-dated sediment core (D13882) obtained from the Atlantic margin of the southwest Iberian Peninsula. The core provides new insights into the dynamics of five ecological pollen-based groups (temperate forests, Mediterranean forests, pinewoods, heathlands and steppe) since the Late Pleistocene (last 13260 cal yr BP). The record shows that the Early Holocene was marked by an increase of temperate and Mediterranean taxa, accompanied by a reduction of steppic plants, reflecting warm and relatively moist conditions when compared with the previous Younger Dryas episode. The timing of this Temperate and Mediterranean Forest (TMF) maximum which reflects optimum conditions of temperature and precipitation is regionally specific, as detected through a comparison of three marine cores across the Iberian Peninsula. Maximum TMF development occurs between 9760 and 7360 cal yr BP in D13882 whereas at a site to the northwest (MD03-2697), the maximum occurs some centuries earlier, and at a southeast site (MD95-2043), the maximum is evident later. The duration of the forest maximum is also progressively longer from northwest to southwest to southeast. A transition from forested ecosystems to heathland over western Iberia characterises the Middle Holocene, starting from around 7360 cal yr BP in D13882, suggesting reduced seasonality and a more Atlantic climate character with moisture evenly available through the year. This contrasts with the aridity recorded in southeast Iberian sites and reflects a stable precipitation gradient between the wetter west and drier east of the Iberian Peninsula. Against this backdrop, pinewoods had a continuous presence, albeit with a trend of steady decline through the Holocene. The time-transgressive pattern in the TMF maximum is best explained by a less favourable moisture balance in the drier southeast sector, which was particularly important during the Early Holocene. Despite the relatively short duration of the Holocene interglacial (one half precession cycle), our observations support a key role for orbital forcing in the temporal expression of major vegetation groups, specifically the decline in summer insolation and the precession-related passage of perihelion season from summer to winter.

Environmental Changes and Cultural Transitions in SW Iberia during the Early-Mid Holocene

“…10 to 7 ka BP seem to confirm this trend. Despite slight differences in the chronologies, this rise of temperate and Mediterranean forest taxa is also registered in other marine cores drilled in the Atlantic margin of southwestern Iberia [101][102][103], as well as in some continental deposits of this area [91]. During this period, two peaks of microcharcoals were recorded and interpreted as episodes of increased regional wildfires at 8.8 and 8.4 ka BP (Figure 7).…”

“…7.5 ka BP, defined by a visible peak of xerophytes and an abrupt drop of mesophilous trees, riparian woodland, and Mediterranean woodland and shrubland, pointing to a rapid decrease of moisture availability (Figures 6e and 7). Diverse episodes of forest contractions were identified between 7.5 and 7 ka BP in different cores of the Atlantic Iberian Margin and the Alborán Sea, as well as in continental sequences [91,[101][102][103]106,107,109]. Diverse palaeorecords from SE Iberia suggest changes towards increased aridity between 7.8-7.3 ka BP [110].…”

“…8.6-8 ka BP, as well as in the Alborán Sea between ca. 8.3-8 ka BP [101][102][103]106,107]. In continental deposits, some forest setbacks and expansion of open-ground taxa were recorded in the Guadiana Valley and the Medina Lagoon at ca.…”

“…Commonly, the expansion of the Neolithic in both the western and eastern sectors of the Gulf of Cádiz is considered to have occurred around 7.5 ka BP [9,12]. Corresponding to the period, different sequences reflect an aridity crisis affecting the vegetation of the SW Atlantic Iberian coast and the Alborán Sea [91,[101][102][103]106,107,109] and the geochemistry dataset reflects a decrease of land moisture (Figure 6e). Some Mesolithic sites exhibit layers of Neolithic occupations (Castelejo, Barranco das Quebradas, Rocha das Gaivotas, Vale Boi, Palmones), although with hiatuses of several hundred years.…”

Reconstructing mesolithic social networks in the Iberian Peninsula using ornaments

“…These latter periods are characterised by a global trend of stabilisation under warmer and wetter climatic conditions (Cacho et al 2001), interrupted by several cooling events, the most notable being the 8.2 kya climatic event (López-Sáez et al 2007). These climatic changes had a transformative effect on the environment with a sea level rise (Peltier 2002) and the expansion of deciduous and thermophilous forests (Carrión et al 2010;Gomes et al 2020).…”